{ "data": { "question": { "questionId": "284", "questionFrontendId": "284", "boundTopicId": null, "title": "Peeking Iterator", "titleSlug": "peeking-iterator", "content": "

Design an iterator that supports the peek operation on an existing iterator in addition to the hasNext and the next operations.

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Implement the PeekingIterator class:

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Note: Each language may have a different implementation of the constructor and Iterator, but they all support the int next() and boolean hasNext() functions.

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Example 1:

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\nInput\n["PeekingIterator", "next", "peek", "next", "next", "hasNext"]\n[[[1, 2, 3]], [], [], [], [], []]\nOutput\n[null, 1, 2, 2, 3, false]\n\nExplanation\nPeekingIterator peekingIterator = new PeekingIterator([1, 2, 3]); // [1,2,3]\npeekingIterator.next();    // return 1, the pointer moves to the next element [1,2,3].\npeekingIterator.peek();    // return 2, the pointer does not move [1,2,3].\npeekingIterator.next();    // return 2, the pointer moves to the next element [1,2,3]\npeekingIterator.next();    // return 3, the pointer moves to the next element [1,2,3]\npeekingIterator.hasNext(); // return False\n
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Constraints:

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\nFollow up: How would you extend your design to be generic and work with all types, not just integer?", "translatedTitle": null, "translatedContent": null, "isPaidOnly": false, "difficulty": "Medium", "likes": 988, "dislikes": 681, "isLiked": null, "similarQuestions": "[{\"title\": \"Binary Search Tree Iterator\", \"titleSlug\": \"binary-search-tree-iterator\", \"difficulty\": \"Medium\", \"translatedTitle\": null}, {\"title\": \"Flatten 2D Vector\", \"titleSlug\": \"flatten-2d-vector\", \"difficulty\": \"Medium\", \"translatedTitle\": null}, {\"title\": \"Zigzag Iterator\", \"titleSlug\": \"zigzag-iterator\", \"difficulty\": \"Medium\", \"translatedTitle\": null}]", "exampleTestcases": "[\"PeekingIterator\",\"next\",\"peek\",\"next\",\"next\",\"hasNext\"]\n[[[1,2,3]],[],[],[],[],[]]", "categoryTitle": "Algorithms", "contributors": [], "topicTags": [ { "name": "Array", "slug": "array", "translatedName": null, "__typename": "TopicTagNode" }, { "name": "Design", "slug": "design", "translatedName": null, "__typename": "TopicTagNode" }, { "name": "Iterator", "slug": "iterator", "translatedName": null, "__typename": "TopicTagNode" } ], "companyTagStats": null, "codeSnippets": [ { "lang": "C++", "langSlug": "cpp", "code": "/*\n * Below is the interface for Iterator, which is already defined for you.\n * **DO NOT** modify the interface for Iterator.\n *\n * class Iterator {\n *\t\tstruct Data;\n * \t\tData* data;\n * public:\n *\t\tIterator(const vector& nums);\n * \t\tIterator(const Iterator& iter);\n *\n * \t\t// Returns the next element in the iteration.\n *\t\tint next();\n *\n *\t\t// Returns true if the iteration has more elements.\n *\t\tbool hasNext() const;\n *\t};\n */\n\nclass PeekingIterator : public Iterator {\npublic:\n\tPeekingIterator(const vector& nums) : Iterator(nums) {\n\t // Initialize any member here.\n\t // **DO NOT** save a copy of nums and manipulate it directly.\n\t // You should only use the Iterator interface methods.\n\t \n\t}\n\t\n // Returns the next element in the iteration without advancing the iterator.\n\tint peek() {\n \n\t}\n\t\n\t// hasNext() and next() should behave the same as in the Iterator interface.\n\t// Override them if needed.\n\tint next() {\n\t \n\t}\n\t\n\tbool hasNext() const {\n\t \n\t}\n};", "__typename": "CodeSnippetNode" }, { "lang": "Java", "langSlug": "java", "code": "// Java Iterator interface reference:\n// https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html\n\nclass PeekingIterator implements Iterator {\n\tpublic PeekingIterator(Iterator iterator) {\n\t // initialize any member here.\n\t \n\t}\n\t\n // Returns the next element in the iteration without advancing the iterator.\n\tpublic Integer peek() {\n \n\t}\n\t\n\t// hasNext() and next() should behave the same as in the Iterator interface.\n\t// Override them if needed.\n\t@Override\n\tpublic Integer next() {\n\t \n\t}\n\t\n\t@Override\n\tpublic boolean hasNext() {\n\t \n\t}\n}", "__typename": "CodeSnippetNode" }, { "lang": "Python", "langSlug": "python", "code": "# Below is the interface for Iterator, which is already defined for you.\n#\n# class Iterator(object):\n# def __init__(self, nums):\n# \"\"\"\n# Initializes an iterator object to the beginning of a list.\n# :type nums: List[int]\n# \"\"\"\n#\n# def hasNext(self):\n# \"\"\"\n# Returns true if the iteration has more elements.\n# :rtype: bool\n# \"\"\"\n#\n# def next(self):\n# \"\"\"\n# Returns the next element in the iteration.\n# :rtype: int\n# \"\"\"\n\nclass PeekingIterator(object):\n def __init__(self, iterator):\n \"\"\"\n Initialize your data structure here.\n :type iterator: Iterator\n \"\"\"\n \n\n def peek(self):\n \"\"\"\n Returns the next element in the iteration without advancing the iterator.\n :rtype: int\n \"\"\"\n \n\n def next(self):\n \"\"\"\n :rtype: int\n \"\"\"\n \n\n def hasNext(self):\n \"\"\"\n :rtype: bool\n \"\"\"\n \n\n# Your PeekingIterator object will be instantiated and called as such:\n# iter = PeekingIterator(Iterator(nums))\n# while iter.hasNext():\n# val = iter.peek() # Get the next element but not advance the iterator.\n# iter.next() # Should return the same value as [val].", "__typename": "CodeSnippetNode" }, { "lang": "Python3", "langSlug": "python3", "code": "# Below is the interface for Iterator, which is already defined for you.\n#\n# class Iterator:\n# def __init__(self, nums):\n# \"\"\"\n# Initializes an iterator object to the beginning of a list.\n# :type nums: List[int]\n# \"\"\"\n#\n# def hasNext(self):\n# \"\"\"\n# Returns true if the iteration has more elements.\n# :rtype: bool\n# \"\"\"\n#\n# def next(self):\n# \"\"\"\n# Returns the next element in the iteration.\n# :rtype: int\n# \"\"\"\n\nclass PeekingIterator:\n def __init__(self, iterator):\n \"\"\"\n Initialize your data structure here.\n :type iterator: Iterator\n \"\"\"\n \n\n def peek(self):\n \"\"\"\n Returns the next element in the iteration without advancing the iterator.\n :rtype: int\n \"\"\"\n \n\n def next(self):\n \"\"\"\n :rtype: int\n \"\"\"\n \n\n def hasNext(self):\n \"\"\"\n :rtype: bool\n \"\"\"\n \n\n# Your PeekingIterator object will be instantiated and called as such:\n# iter = PeekingIterator(Iterator(nums))\n# while iter.hasNext():\n# val = iter.peek() # Get the next element but not advance the iterator.\n# iter.next() # Should return the same value as [val].", "__typename": "CodeSnippetNode" }, { "lang": "C", "langSlug": "c", "code": "/*\n *\tstruct Iterator {\n *\t\t// Returns true if the iteration has more elements.\n *\t\tbool (*hasNext)();\n *\n * \t\t// Returns the next element in the iteration.\n *\t\tint (*next)();\n *\t};\n */\n\nstruct PeekingIterator {\n \n};\n\nstruct PeekingIterator* Constructor(struct Iterator* iter) {\n struct PeekingIterator* piter = malloc(sizeof(struct PeekingIterator));\n piter->iterator = iter;\n piter->hasPeeked = false;\n return piter;\n}\n\nint peek(struct PeekingIterator* obj) {\n \n}\n\nint next(struct PeekingIterator* obj) {\n \n}\n\nbool hasNext(struct PeekingIterator* obj) {\n \n}\n\n/**\n * Your PeekingIterator struct will be instantiated and called as such:\n * PeekingIterator* obj = peekingIteratorCreate(arr, arrSize);\n * int param_1 = peek(obj);\n * int param_2 = next(obj);\n * bool param_3 = hasNext(obj);\n * peekingIteratorFree(obj);\n*/", "__typename": "CodeSnippetNode" }, { "lang": "C#", "langSlug": "csharp", "code": "// C# IEnumerator interface reference:\n// https://docs.microsoft.com/en-us/dotnet/api/system.collections.ienumerator?view=netframework-4.8\n\nclass PeekingIterator {\n // iterators refers to the first element of the array.\n public PeekingIterator(IEnumerator iterator) {\n // initialize any member here.\n }\n \n // Returns the next element in the iteration without advancing the iterator.\n public int Peek() {\n \n }\n \n // Returns the next element in the iteration and advances the iterator.\n public int Next() {\n \n }\n \n // Returns false if the iterator is refering to the end of the array of true otherwise.\n public bool HasNext() {\n\t\t\n }\n}", "__typename": "CodeSnippetNode" }, { "lang": "JavaScript", "langSlug": "javascript", "code": "/**\n * // This is the Iterator's API interface.\n * // You should not implement it, or speculate about its implementation.\n * function Iterator() {\n * @ return {number}\n * this.next = function() { // return the next number of the iterator\n * ...\n * }; \n *\n * @return {boolean}\n * this.hasNext = function() { // return true if it still has numbers\n * ...\n * };\n * };\n */\n\n/**\n * @param {Iterator} iterator\n */\nvar PeekingIterator = function(iterator) {\n \n};\n\n/**\n * @return {number}\n */\nPeekingIterator.prototype.peek = function() {\n \n};\n\n/**\n * @return {number}\n */\nPeekingIterator.prototype.next = function() {\n \n};\n\n/**\n * @return {boolean}\n */\nPeekingIterator.prototype.hasNext = function() {\n \n};\n\n/** \n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = new PeekingIterator(arr)\n * var param_1 = obj.peek()\n * var param_2 = obj.next()\n * var param_3 = obj.hasNext()\n */", "__typename": "CodeSnippetNode" }, { "lang": "Ruby", "langSlug": "ruby", "code": "# Below is the interface for Iterator, which is already defined for you.\n#\n# class Iterator\n# \tdef initialize(v)\n# \n# end\n#\n# def hasNext()\n#\t\tReturns true if the iteration has more elements.\n# end\n#\n# def next()\n# \tReturns the next element in the iteration.\n# end\n# end\n\nclass PeekingIterator\n # @param {Iterator} iter\n def initialize(iter)\n \t\n end\n \n # Returns true if the iteration has more elements.\n # @return {boolean}\n def hasNext()\n \t\n end\n \n # Returns the next element in the iteration.\n # @return {integer}\n def next()\n \t\n end\n \n # Returns the next element in the iteration without advancing the iterator.\n # @return {integer}\n def peek()\n \t\n end\nend", "__typename": "CodeSnippetNode" }, { "lang": "Swift", "langSlug": "swift", "code": "// Swift IndexingIterator refernence:\n// https://developer.apple.com/documentation/swift/indexingiterator\n\nclass PeekingIterator {\n init(_ arr: IndexingIterator>) {\n \n }\n \n func next() -> Int {\n \n }\n \n func peek() -> Int {\n \n }\n \n func hasNext() -> Bool {\n \n }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * let obj = PeekingIterator(arr)\n * let ret_1: Int = obj.next()\n * let ret_2: Int = obj.peek()\n * let ret_3: Bool = obj.hasNext()\n */", "__typename": "CodeSnippetNode" }, { "lang": "Go", "langSlug": "golang", "code": "/* Below is the interface for Iterator, which is already defined for you.\n *\n * type Iterator struct {\n * \n * }\n *\n * func (this *Iterator) hasNext() bool {\n *\t\t// Returns true if the iteration has more elements.\n * }\n *\n * func (this *Iterator) next() int {\n *\t\t// Returns the next element in the iteration.\n * }\n */\n\ntype PeekingIterator struct {\n \n}\n\nfunc Constructor(iter *Iterator) *PeekingIterator {\n \n}\n\nfunc (this *PeekingIterator) hasNext() bool {\n \n}\n\nfunc (this *PeekingIterator) next() int {\n \n}\n\nfunc (this *PeekingIterator) peek() int {\n \n}", "__typename": "CodeSnippetNode" }, { "lang": "Scala", "langSlug": "scala", "code": "// Scala Iterator reference:\n// https://www.scala-lang.org/api/2.12.2/scala/collection/Iterator.html\n\nclass PeekingIterator(_iterator: Iterator[Int]) {\n def peek(): Int = {\n \n }\n \n def next(): Int = {\n \n }\n \n def hasNext(): Boolean = {\n \n }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = new PeekingIterator(arr)\n * var param_1 = obj.next()\n * var param_2 = obj.peek()\n * var param_3 = obj.hasNext()\n */", "__typename": "CodeSnippetNode" }, { "lang": "Kotlin", "langSlug": "kotlin", "code": "// Kotlin Iterator reference:\n// https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-iterator/\n\nclass PeekingIterator(iterator:Iterator):Iterator {\n fun peek(): Int {\n \t\n }\n \n override fun next(): Int {\n \n }\n \n override fun hasNext(): Boolean {\n \n }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = PeekingIterator(arr)\n * var param_1 = obj.next()\n * var param_2 = obj.peek()\n * var param_3 = obj.hasNext()\n */", "__typename": "CodeSnippetNode" }, { "lang": "PHP", "langSlug": "php", "code": "// PHP ArrayIterator reference:\n// https://www.php.net/arrayiterator\n\nclass PeekingIterator {\n /**\n * @param ArrayIterator $arr\n */\n function __construct($arr) {\n \n }\n \n /**\n * @return Integer\n */\n function next() {\n \n }\n \n /**\n * @return Integer\n */\n function peek() {\n \n }\n \n /**\n * @return Boolean\n */\n function hasNext() {\n \n }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * $obj = PeekingIterator($arr);\n * $ret_1 = $obj->next();\n * $ret_2 = $obj->peek();\n * $ret_3 = $obj->hasNext();\n */", "__typename": "CodeSnippetNode" }, { "lang": "TypeScript", "langSlug": "typescript", "code": "/**\n * // This is the Iterator's API interface.\n * // You should not implement it, or speculate about its implementation\n * class Iterator {\n * hasNext(): boolean {}\n *\n * next(): number {}\n * }\n */\n\nclass PeekingIterator {\n constructor(iterator: Iterator) {\n\n }\n\n peek(): number {\n\n }\n\n next(): number {\n\n }\n\n hasNext(): boolean {\n\n }\n}\n\n/**\n * Your PeekingIterator object will be instantiated and called as such:\n * var obj = new PeekingIterator(iterator)\n * var param_1 = obj.peek()\n * var param_2 = obj.next()\n * var param_3 = obj.hasNext()\n */", "__typename": "CodeSnippetNode" } ], "stats": "{\"totalAccepted\": \"155.1K\", \"totalSubmission\": \"290K\", \"totalAcceptedRaw\": 155080, \"totalSubmissionRaw\": 289953, \"acRate\": \"53.5%\"}", "hints": [ "Think of \"looking ahead\". You want to cache the next element.", "Is one variable sufficient? Why or why not?", "Test your design with call order of peek() before next() vs next() before peek().", "For a clean implementation, check out Google's guava library source code." ], "solution": { "id": "879", "canSeeDetail": false, "paidOnly": true, "hasVideoSolution": false, "paidOnlyVideo": true, "__typename": "ArticleNode" }, "status": null, "sampleTestCase": "[\"PeekingIterator\",\"next\",\"peek\",\"next\",\"next\",\"hasNext\"]\n[[[1,2,3]],[],[],[],[],[]]", "metaData": "{\n \"name\": \"PeekingIterator\",\n \"params\": [\n {\n \"name\": \"nums\",\n \"type\": \"list\"\n },\n {\n \"name\": \"commands\",\n \"type\": \"list\"\n }\n ],\n \"return\": {\n \"type\": \"list\",\n \"dealloc\": true\n },\n \"languages\": [\n \"cpp\",\n \"java\",\n \"python\",\n \"python3\",\n \"csharp\",\n \"golang\",\n \"ruby\",\n \"kotlin\",\n \"scala\",\n \"swift\",\n \"php\",\n \"c\",\n \"javascript\",\n \"typescript\"\n ],\n \"manual\": true\n}", "judgerAvailable": true, "judgeType": "large", "mysqlSchemas": [], "enableRunCode": true, "enableTestMode": false, "enableDebugger": true, "envInfo": "{\"cpp\": [\"C++\", \"

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