leetcode402 移掉k位数字

 1 class Solution
 2 {
 3 public:
 4     string removeKdigits(string num, int k)
 5     {
 6         stack<char> s;
 7         int n = num.size(), r = k;
 8         for (int i = 0; i < n; i++)
 9         {
10             char x = num[i];
11             while (r and !s.empty() and x < s.top())
12             {
13                 r--; s.pop();
14             }
15             s.push(x);
16         }
17         string res = "";
18         while (!s.empty())
19         {
20             res += s.top(); s.pop();
21         }
22         reverse(res.begin(), res.end());
23         res = res.substr(0, n - k);
24         int p = 0, l = res.length();
25         while (p < l and res[p] == ‘0‘) p++;
26         return p == l ? "0": res.substr(p, l - p);
27     }
28 };