{ "data": { "question": { "questionId": "1491", "questionFrontendId": "1375", "categoryTitle": "Algorithms", "boundTopicId": 132469, "title": "Number of Times Binary String Is Prefix-Aligned", "titleSlug": "number-of-times-binary-string-is-prefix-aligned", "content": "
You have a 1-indexed binary string of length n
where all the bits are 0
initially. We will flip all the bits of this binary string (i.e., change them from 0
to 1
) one by one. You are given a 1-indexed integer array flips
where flips[i]
indicates that the bit at index i
will be flipped in the ith
step.
A binary string is prefix-aligned if, after the ith
step, all the bits in the inclusive range [1, i]
are ones and all the other bits are zeros.
Return the number of times the binary string is prefix-aligned during the flipping process.
\n\n\n
Example 1:
\n\n\nInput: flips = [3,2,4,1,5]\nOutput: 2\nExplanation: The binary string is initially "00000".\nAfter applying step 1: The string becomes "00100", which is not prefix-aligned.\nAfter applying step 2: The string becomes "01100", which is not prefix-aligned.\nAfter applying step 3: The string becomes "01110", which is not prefix-aligned.\nAfter applying step 4: The string becomes "11110", which is prefix-aligned.\nAfter applying step 5: The string becomes "11111", which is prefix-aligned.\nWe can see that the string was prefix-aligned 2 times, so we return 2.\n\n\n
Example 2:
\n\n\nInput: flips = [4,1,2,3]\nOutput: 1\nExplanation: The binary string is initially "0000".\nAfter applying step 1: The string becomes "0001", which is not prefix-aligned.\nAfter applying step 2: The string becomes "1001", which is not prefix-aligned.\nAfter applying step 3: The string becomes "1101", which is not prefix-aligned.\nAfter applying step 4: The string becomes "1111", which is prefix-aligned.\nWe can see that the string was prefix-aligned 1 time, so we return 1.\n\n\n
\n
Constraints:
\n\nn == flips.length
1 <= n <= 5 * 104
flips
is a permutation of the integers in the range [1, n]
.给你一个长度为 n
、下标从 1 开始的二进制字符串,所有位最开始都是 0
。我们会按步翻转该二进制字符串的所有位(即,将 0
变为 1
)。
给你一个下标从 1 开始的整数数组 flips
,其中 flips[i]
表示对应下标 i
的位将会在第 i
步翻转。
二进制字符串 前缀一致 需满足:在第 i
步之后,在 闭 区间 [1, i]
内的所有位都是 1 ,而其他位都是 0 。
返回二进制字符串在翻转过程中 前缀一致 的次数。
\n\n\n\n
示例 1:
\n\n\n输入:flips = [3,2,4,1,5]\n输出:2\n解释:二进制字符串最开始是 \"00000\" 。\n执行第 1 步:字符串变为 \"00100\" ,不属于前缀一致的情况。\n执行第 2 步:字符串变为 \"01100\" ,不属于前缀一致的情况。\n执行第 3 步:字符串变为 \"01110\" ,不属于前缀一致的情况。\n执行第 4 步:字符串变为 \"11110\" ,属于前缀一致的情况。\n执行第 5 步:字符串变为 \"11111\" ,属于前缀一致的情况。\n在翻转过程中,前缀一致的次数为 2 ,所以返回 2 。\n\n\n
示例 2:
\n\n\n输入:flips = [4,1,2,3]\n输出:1\n解释:二进制字符串最开始是 \"0000\" 。\n执行第 1 步:字符串变为 \"0001\" ,不属于前缀一致的情况。\n执行第 2 步:字符串变为 \"1001\" ,不属于前缀一致的情况。\n执行第 3 步:字符串变为 \"1101\" ,不属于前缀一致的情况。\n执行第 4 步:字符串变为 \"1111\" ,属于前缀一致的情况。\n在翻转过程中,前缀一致的次数为 1 ,所以返回 1 。\n\n
\n\n
提示:
\n\nn == flips.length
1 <= n <= 5 * 104
flips
是范围 [1, n]
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