leetcode 402. Remove K Digits

Given a non-negative integer num represented as a string, remove k digits from the number so that the new number is the smallest possible.

Note:
The length of num is less than 10002 and will be ≥ k.
The given num does not contain any leading zero.
Example 1:

Input: num = "1432219", k = 3
Output: "1219"
Explanation: Remove the three digits 4, 3, and 2 to form the new number 1219 which is the smallest.
Example 2:

Input: num = "10200", k = 1
Output: "200"
Explanation: Remove the leading 1 and the number is 200. Note that the output must not contain leading zeroes.
Example 3:

Input: num = "10", k = 2
Output: "0"
Explanation: Remove all the digits from the number and it is left with nothing which is 0.

题目大意：
删除K个数字，使得最后得到的数字尽量小。
思路：
贪心，当遇到比队列尾部小的，就试着往里面放，知道放不进去为止或者队列不能再弹出为止。
代码如下：

class Solution {
public:
    string removeKdigits(string num, int k) {
        string t = "";
        vector<char> s;
        int x = num.size() - k;
        for (int i = 0; i < num.size(); ++i) {
            if (s.size() == 0) s.push_back(num[i]);
            else {
                if (num[i] < s.back()) {
                    while (!s.empty() && (s.back() > num[i]) && ((num.size() - i + s.size()) > x)) {
                        s.pop_back();
                    }
                }
                s.push_back(num[i]);
            }
        }
        for (int i = 0; i < x; ++i) {
            if (s[i] == '0' && t == "") continue;
            else t += s[i];
        }
        if (t == "" || x == 0) t = '0';
        return t;
    }
};

注意边界...我好菜啊，改了半天