402. Remove K Digits

• Difficulty: Medium

• Topics: Stack, Greedy

• Similar Questions:

  • Create Maximum Number

  • Monotone Increasing Digits

Problem:

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.

Solutions:

class Solution {
public:
    string removeKdigits(string num, int k) {
        vector<char> stk;
        num.push_back('0' - 1);
        for (auto& c : num) {
            while (k > 0 && !stk.empty() && stk.back() > c) {
                stk.pop_back();
                --k;
            }
            stk.push_back(c);
        }
        stk.pop_back();
        int start = 0;
        for (start = 0; start < stk.size() && stk[start] == '0'; ++start)  ;
        if (start == stk.size())    return "0";
        return string(stk.begin() + start, stk.end());        
    }
};