738. Monotone Increasing Digits

Given a non-negative integer N, find the largest number that is less than or equal to N with monotone increasing digits.

(Recall that an integer has monotone increasing digits if and only if each pair of adjacent digits x and y satisfy x <= y.)

Example 1:Input: N = 10Output: 9Example 2:Input: N = 1234Output: 1234Example 3:Input: N = 332Output: 299

这题暴力解法就是一个个数check，超时。
贪心算法，构造需要的解去试。
key point就是，如果已知现有的部分是‘123’，下一个数字最大可以是多少了？比如5，那么就要保证1235…5 <= N。在这个要求下去找每一位可能的数字就行。
代码：

class Solution {public:    int monotoneIncreasingDigits(int N) {        string res = "";        string num = to_string(N);        for (int i = 0; i < num.size(); i++) {            int flag = false;            for (char c = '1'; c <= '9'; c++) {                string temp= res;                while (temp.size() < num.size()) temp += c;                if (temp > num) {                    res += c - 1;                    flag = true;                    break;                }            }            if (flag == false) res += '9';        }        return stoi(res);    }};