{ "data": { "question": { "questionId": "3184", "questionFrontendId": "2926", "categoryTitle": "Algorithms", "boundTopicId": 2510668, "title": "Maximum Balanced Subsequence Sum", "titleSlug": "maximum-balanced-subsequence-sum", "content": "
You are given a 0-indexed integer array nums
.
A subsequence of nums
having length k
and consisting of indices i0 < i1 < ... < ik-1
is balanced if the following holds:
nums[ij] - nums[ij-1] >= ij - ij-1
, for every j
in the range [1, k - 1]
.A subsequence of nums
having length 1
is considered balanced.
Return an integer denoting the maximum possible sum of elements in a balanced subsequence of nums
.
A subsequence of an array is a new non-empty array that is formed from the original array by deleting some (possibly none) of the elements without disturbing the relative positions of the remaining elements.
\n\n\n
Example 1:
\n\n\nInput: nums = [3,3,5,6]\nOutput: 14\nExplanation: In this example, the subsequence [3,5,6] consisting of indices 0, 2, and 3 can be selected.\nnums[2] - nums[0] >= 2 - 0.\nnums[3] - nums[2] >= 3 - 2.\nHence, it is a balanced subsequence, and its sum is the maximum among the balanced subsequences of nums.\nThe subsequence consisting of indices 1, 2, and 3 is also valid.\nIt can be shown that it is not possible to get a balanced subsequence with a sum greater than 14.\n\n
Example 2:
\n\n\nInput: nums = [5,-1,-3,8]\nOutput: 13\nExplanation: In this example, the subsequence [5,8] consisting of indices 0 and 3 can be selected.\nnums[3] - nums[0] >= 3 - 0.\nHence, it is a balanced subsequence, and its sum is the maximum among the balanced subsequences of nums.\nIt can be shown that it is not possible to get a balanced subsequence with a sum greater than 13.\n\n\n
Example 3:
\n\n\nInput: nums = [-2,-1]\nOutput: -1\nExplanation: In this example, the subsequence [-1] can be selected.\nIt is a balanced subsequence, and its sum is the maximum among the balanced subsequences of nums.\n\n\n
\n
Constraints:
\n\n1 <= nums.length <= 105
-109 <= nums[i] <= 109
给你一个下标从 0 开始的整数数组 nums
。
nums
一个长度为 k
的 子序列 指的是选出 k
个 下标 i0 < i1 < ... < ik-1
,如果这个子序列满足以下条件,我们说它是 平衡的 :
[1, k - 1]
内的所有 j
,nums[ij] - nums[ij-1] >= ij - ij-1
都成立。nums
长度为 1
的 子序列 是平衡的。
请你返回一个整数,表示 nums
平衡 子序列里面的 最大元素和 。
一个数组的 子序列 指的是从原数组中删除一些元素(也可能一个元素也不删除)后,剩余元素保持相对顺序得到的 非空 新数组。
\n\n\n\n
示例 1:
\n\n\n输入:nums = [3,3,5,6]\n输出:14\n解释:这个例子中,选择子序列 [3,5,6] ,下标为 0 ,2 和 3 的元素被选中。\nnums[2] - nums[0] >= 2 - 0 。\nnums[3] - nums[2] >= 3 - 2 。\n所以,这是一个平衡子序列,且它的和是所有平衡子序列里最大的。\n包含下标 1 ,2 和 3 的子序列也是一个平衡的子序列。\n最大平衡子序列和为 14 。\n\n
示例 2:
\n\n\n输入:nums = [5,-1,-3,8]\n输出:13\n解释:这个例子中,选择子序列 [5,8] ,下标为 0 和 3 的元素被选中。\nnums[3] - nums[0] >= 3 - 0 。\n所以,这是一个平衡子序列,且它的和是所有平衡子序列里最大的。\n最大平衡子序列和为 13 。\n\n\n
示例 3:
\n\n\n输入:nums = [-2,-1]\n输出:-1\n解释:这个例子中,选择子序列 [-1] 。\n这是一个平衡子序列,而且它的和是 nums 所有平衡子序列里最大的。\n\n\n
\n\n
提示:
\n\n1 <= nums.length <= 105
-109 <= nums[i] <= 109
dp[x]
represent the maximum sum of a balanced subsequence ending at x
.",
"Rewriting the formula nums[ij] - nums[ij-1] >= ij - ij-1
gives nums[ij] - ij >= nums[ij-1] - ij-1
.",
"So, for some index x
, we need to find an index y
, y < x
, such that dp[x] = nums[x] + dp[y]
is maximized, and nums[x] - x >= nums[y] - y
.",
"There are many ways to achieve this. One method involves sorting the values of nums[x] - x
for all indices x
and using a segment/Fenwick tree with coordinate compression.",
"Hence, using a dictionary or map, let's call it dict
, where dict[nums[x] - x]
represents the position of the value, nums[x] - x
, in the segment tree.",
"The tree is initialized with zeros initially.",
"For indices x
in order from [0, n - 1]
, dp[x] = max(nums[x]
, nums[x]
+ the maximum query from the tree in the range [0, dict[nums[x] - x]])
, and if dp[x]
is greater than the value in the tree at position dict[nums[x] - x]
, we update the value in the tree.",
"The answer to the problem is the maximum value in dp
."
],
"solution": null,
"status": null,
"sampleTestCase": "[3,3,5,6]",
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