動機

可以當成有一個bar，從左邊推到右邊

Problem

You are given a string s, a split is called good if you can split s into 2 non-empty strings p and q where its concatenation is equal to s and the number of distinct letters in p and q are the same.

Return the number of good splits you can make in s.

 

Example 1:

Input: s = "aacaba"Output: 2Explanation: There are 5 ways to split "aacaba" and 2 of them are good. ("a", "acaba") Left string and right string contains 1 and 3 different letters respectively.("aa", "caba") Left string and right string contains 1 and 3 different letters respectively.("aac", "aba") Left string and right string contains 2 and 2 different letters respectively (good split).("aaca", "ba") Left string and right string contains 2 and 2 different letters respectively (good split).("aacab", "a") Left string and right string contains 3 and 1 different letters respectively.

Example 2:

Input: s = "abcd"Output: 1Explanation: Split the string as follows ("ab", "cd").

Example 3:

Input: s = "aaaaa"Output: 4Explanation: All possible splits are good.

Example 4:

Input: s = "acbadbaada"Output: 2

 

Constraints:

  • s contains only lowercase English letters.
  • 1 <= s.length <= 10^5

Sol

class Solution:
    def numSplits(self, s: str) -> int:
        left ,right = set(), Counter(s)
        
        ret = 0
        for c in s:
            left.add(c)
            right[c] -= 1
            
            if not right[c]:
                del right[c]
            if len(left) == len(right):
                ret += 1
            
        return ret