LeetCode 926. Flip String to Monotone Increasing

原题链接在这里：https://leetcode.com/problems/flip-string-to-monotone-increasing/

题目：

A string of '0's and '1's is monotone increasing if it consists of some number of '0's (possibly 0), followed by some number of '1's (also possibly 0.)

We are given a string S of '0's and '1's, and we may flip any '0' to a '1' or a '1' to a '0'.

Return the minimum number of flips to make S monotone increasing.

Example 1:

Input: "00110"
Output: 1
Explanation: We flip the last digit to get 00111.

Example 2:

Input: "010110"
Output: 2
Explanation: We flip to get 011111, or alternatively 000111.

Example 3:

Input: "00011000"
Output: 2
Explanation: We flip to get 00000000. 

Note:

1. 1 <= S.length <= 20000
2. S only consists of '0' and '1' characters.

题解：

Say up to i, we have already flip as minimum number of flips needed.

Then it comes to i + 1. If it is 1, then there is nothing changed.

If it is 0, there are 2 options, either convert it to 1, then it needs flip + 1.

Or convert all previous 1 into 0, then it needs oneCount. Take the minimum.

Time Complexity: O(n). n = S.length().

Space: O(1).

AC Java:

class Solution {
    public int minFlipsMonoIncr(String S) {
        int oneCount = 0;
        int flip = 0;
        for(int i = 0; i < S.length(); i++){
            if(S.charAt(i) == '1'){
                oneCount++;
            }else{
                flip = Math.min(flip + 1, oneCount);
            }
        }
        
        return flip;
    }
}