Press "Enter" to skip to content

花花酱 LeetCode 1573. Number of Ways to Split a String

Given a binary string s (a string consisting only of ‘0’s and ‘1’s), we can split s into 3 non-empty strings s1, s2, s3 (s1+ s2+ s3 = s).

Return the number of ways s can be split such that the number of characters ‘1’ is the same in s1, s2, and s3.

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

Example 1:

Input: s = "10101"
Output: 4
Explanation: There are four ways to split s in 3 parts where each part contain the same number of letters '1'.

Example 2:

Input: s = "1001"
Output: 0

Example 3:

Input: s = "0000"
Output: 3
Explanation: There are three ways to split s in 3 parts.

Example 4:

Input: s = "100100010100110"
Output: 12


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

Solution: Counting

Count how many ones in the binary string as T, if not a factor of 3, then there is no answer.

Count how many positions that have prefix sum of T/3 as l, and how many positions that have prefix sum of T/3*2 as r.

Ans = l * r

But we need to special handle the all zero cases, which equals to C(n-2, 2) = (n – 1) * (n – 2) / 2

Time complexity: O(n)
Space complexity: O(1)



One pass: Space complexity: O(n)


If you like my articles / videos, donations are welcome.

Buy anything from Amazon to support our website


Be First to Comment

Leave a Reply