LintCode 743. 单调递增的数字

 1 class Solution {
 2 public:
 3     /**
 4      * @param num: a non-negative integer N
 5      * @return: the largest number that is less than or equal to N with monotone increasing digits.
 6      */
 7     int monotoneDigits(int num) {
 8         // write your code here
 9         string s = to_string(num);
10         char before = ‘9‘;
11         int index = s.size()-1;
12         for(int i = s.size()-1; i>=0; --i){//从后往前找到增加最后一对上升数对
13             if(s[i]>before) index = i;
14             before = s[i];
15         }
16         if(index == s.size()-1) return num;//index没变表示全部升序排列，返回原数
17         while(index>0 && s[index-1] == s[index]) index--;//关键一步，否则会出错，注意越界
18         s[index]--;
19         for(int j = index+1; j<s.size(); ++j) s[j]=‘9‘;//后面的数全部转变为9
20         return stoi(s);//用stoi省去了头部可能的去0操作
21     }
22 };