【leetcode Weekly Contest 107】926. Flip String to Monotone Increasing

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.

 思路：第一趟遍历，记录字符串中0的个数，即当全部翻转为1需要的次数。

             第二次遍历，从零位开始，每当这位为1，将sum+1,这位为0，sum-1，即基于之前的sum，将第i位（包括i）之前为0，之后为1需要翻转的次数，每次的sum都与前一个sum正好差1，记录出所有sum中最小的即为结果。

class Solution {
public:
    int minFlipsMonoIncr(string S) {
        int sum=0,smax=20005;
        int l=S.length();
        for(int i=0;i<l;i++)
        {
            if(S[i]!='1') sum++;
        }
        smax=sum;
        for(int i=0;i<l;i++)
        {
            if(S[i]=='0') sum--;
            else sum++;
            if(sum<smax) smax=sum;
        }
        return smax;
    }
};