[LeetCode] 926. Flip String to Monotone Increasing

 Problem : https://leetcode.com/problems/flip-string-to-monotone-increasing/

One valid monotone-increasing can either end by '0'  or '1'.

For every s[i], consider the number of flip may need by appending it to the monotone-increasing ended by flipping s[i-1].

class Solution:
    def minFlipsMonoIncr(self, s: str) -> int:
        N = len(s)
        
        end_by_zero = 0 if s[0] == '0' else 1
        end_by_one = 0 if s[1] == '1' else 0
        
        for i in range(1, N):
            if s[i] == '0':
                # no need to flip to expand valid monotone increasing end by 0
                # end_by_zero = end_by_zero
                
                # flip 0 to 1 to expand valid monotone increasing end by 1
                end_by_one = end_by_one + 1
            elif s[i] == '1':
                # no need to flip to expand valid monotone increasing end by 1
                end_by_one = min(end_by_zero, end_by_one)
                
                # flip 1 to 0 to expand valid monotone increasing end by 1
                end_by_zero = end_by_zero + 1         
        
        return min(end_by_zero, end_by_one)