1653 - Minimum Deletions to Make String Balanced\.

Medium

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:

• <code>1 <= s.length <= 10⁵</code>

• s[i] is 'a' or 'b'.