[LeetCode] Remove K Digits 去掉K位数字

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.

这道题让我们将给定的数字去掉k位，要使得留下来的数字最小，这题跟LeetCode上之前那道Create Maximum Number有些类似，可以借鉴其中的思路，如果n是num的长度，我们要去除k个，那么需要剩下n-k个，我们开始遍历给定数字num的每一位，对于当前遍历到的数字c，进行如下while循环，如果res不为空，且k大于0，且res的最后一位大于c，那么我们应该将res的最后一位移去，且k自减1。当跳出while循环后，我们将c加入res中，最后我们将res的大小重设为n-k。根据题目中的描述，可能会出现"0200"这样不符合要求的情况，所以我们用一个while循环来去掉前面的所有0，然后返回时判断是否为空，为空则返回“0”，参见代码如下：

class Solution {
public:
    string removeKdigits(string num, int k) {
        string res = "";
        int n = num.size(), keep = n - k;
        for (char c : num) {
            while (k && res.size() && res.back() > c) {
                res.pop_back();
                --k;
            }
            res.push_back(c);
        }
        res.resize(keep);
        while (!res.empty() && res[0] == '0') res.erase(res.begin());
        return res.empty() ? "0" : res;
    }
};