leetcode 402删除k个数字

题目： 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. 题解： 维持一个单调数列，只要使从栈底到栈顶是单调递增的，便可以使得剩下的数字最小 Python代码： def removeKdigits(self, num, k): “”" :type num: str :type k: int :rtype: str “”" out=[] for d in num: while k and out and out[-1]>d: out.pop() k-=1 out.append(d) return ‘’.join(out[:-k or None]).lstrip(‘0’) or ‘0’

**C++代码：**
       string removeKdigits(string num, int k) {
    string ans="";
    for(auto c:num)
    {
        while(ans.length()&&ans.back()>c&&k)
        {
            --k;
            ans.pop_back();
        }
        if(ans.length()||c!=0)
            ans.push_back(c);
    }
    while(ans.length()&&k){
        ans.pop_back();
        --k;
    }
    return ans.empty()?"0":ans;
}