leetcode 738. 单调递增的数字

class Solution {
public:
    int monotoneIncreasingDigits(int N) {
        string ch = to_string(N);
        int index = 0,i;
        for(i=1;i<ch.size();i++){
            if(ch[i]>ch[i-1])   index=i;
            else if(ch[i]==ch[i-1]) continue;
            else break;
        }
        if(i==ch.size()) return N;
        ch[index]=ch[index]-1;
        for(index++;index<ch.size();index++){
            ch[index]='9';
        }
        return atoi(ch.c_str());
    }
};