# Question

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'​​.


# Algorithm

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].

Java