[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.

分析

这道题可以用单调递增序列来做，例如：123456，去除3位，应当去除后三位，因为是单调递增的。

如果是654321，那么需要扣除前三位后可以满足。

举例： 135426，k=3

135，next=4

134，next=2

12,next=6

得到126

Code

class Solution {
public:
    string removeKdigits(string num, int k) {
        string s;
        int length = num.size();
        int r = 0;
        for (int i = 0; i < length; i ++)
        {
            if (r == k)
            {
                s.push_back(num[i]);
            }
            else
            {
                while (!s.empty() && r < k)
                {
                    if (s.back()> num[i])
                    {
                        r ++;
                        s.pop_back();
                    }
                    else
                    {
                        break;
                    }
                }
                s.push_back(num[i]);
            }
        }
        
        s = s.substr(0, length - k);
        while (s.front() == '0' && s.size() > 1)
        {
            s.erase(s.begin());
        }
        
        return s.size() == 0 ? "0" : s;
    }
};

运行效率

Runtime: 4 ms, faster than 99.95% of C++ online submissions for Remove K Digits.

Memory Usage: 9.3 MB, less than 76.19% of C++ online submissions forRemove K Digits.