1513. Number of Substrings With Only 1s

Given a binary string s (a string consisting only of ‘0’ and ‘1’s).

Return the number of substrings with all characters 1’s.

Since the answer may be too large, return it modulo 10^9 + 7.

Example 1:

Input: s = "0110111"
Output: 9
Explanation: There are 9 substring in total with only 1's characters.
"1" -> 5 times.
"11" -> 3 times.
"111" -> 1 time.

Example 2:

Input: s = "101"
Output: 2
Explanation: Substring "1" is shown 2 times in s.

Example 3:

Input: s = "111111"
Output: 21
Explanation: Each substring contains only 1's characters.

Example 4:

Input: s = "000"
Output: 0

Constraints:

• s[i] == '0' or s[i] == '1'
• 1 <= s.length <= 10^5

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给定一个字符串，问有多少个连续为1的子串。

简单题，先找出最长连续的1，假设长度为n，则这个n长的串能形成(n+1)n/2个1的子串，累加所有这样的子串和即可。代码如下：

const long long kMOD = 1e9 + 7;

class Solution {
	long long CalAccSum(long long l) {
		return (l %kMOD)* ((l + 1) % kMOD) / 2;
	}
public:
	int numSub(string s) {

		long long ans = 0;
		int i = 0, j = 0, n = s.size();
		while (i < n) {
			while (i < n&&s[i] == '0')++i;
			if (i >= n)break;
			j = i + 1;
			while (j < n&&s[j] == '1')++j;
			int m = j - i;
			ans += CalAccSum(m) % kMOD;
			i = j;
		}
		return ans % kMOD;
	}
};

本代码提交AC，用时36MS。