題目鏈接:https://leetcode.com/problems/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
思路:其基本思想是利用棧儘量維持一個遞增的序列,也就是說將字符串中字符依次入棧,如果當前字符串比棧頂元素小,並且還可以繼續刪除元素,那麼就將棧頂元素刪掉,這樣可以保證將當前元素加進去一定可以得到一個較小的序列.也可以算是一個貪心思想.最後我們只取前len-k個元素構成一個序列即可,如果這樣得到的是一個空串那就手動返回0.還有一個需要注意的是字符串首字符不爲0
代碼如下:
class Solution {
public:
string removeKdigits(string num, int k) {
string ans;
int n = k, len = num.size(), cnt = 0;
for(auto val: num)
{
while(!ans.empty() && n > 0 && val < ans.back())
{
n--;
ans.pop_back();
}
ans.push_back(val);
}
while(ans[cnt]=='0') cnt++;
ans = ans.substr(cnt, len-k-cnt);
return !ans.size()?"0":ans;
}
};