Formatted question description: https://leetcode.ca/all/1653.html
1653. Minimum Deletions to Make String Balanced You are given a string s consisting only of characters 'a' and 'b'. You can delete any number of characters in s to make s balanced. s is balanced if there is no pair of indices (i,j) such that i < j and s[i] = 'b' and s[j]= 'a'. Return the minimum number of deletions needed to make s balanced. Example 1: Input: s = "aababbab" Output: 2 Explanation: You can either: Delete the characters at 0-indexed positions 2 and 6 ("aababbab" -> "aaabbb"), or Delete the characters at 0-indexed positions 3 and 6 ("aababbab" -> "aabbbb"). Example 2: Input: s = "bbaaaaabb" Output: 2 Explanation: The only solution is to delete the first two characters. Constraints: 1 <= s.length <= 105 s[i] is 'a' or 'b'.
We only need to enumerate the demarcation points.
For example, we enumerate s[0:k] are all a, and s[k+1:len-1] are all b, then only need to know how many b are in s[0:k], and how many a in s[k+1:len-1].