共计 1042 个字符，预计需要花费 3 分钟才能阅读完成。
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 = 3Output: “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 = 1Output: “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 = 2Output: “0”Explanation: Remove all the digits from the number and it is left with nothing which is 0.
Solution
class Solution {
public String removeKdigits(String num, int k) {
Deque<Character> stack = new ArrayDeque<>();
int n = num.length();
if (k >= n) return “0”;
for (int i = 0; i < n; i++) {
while (k > 0 && !stack.isEmpty() && stack.peek() > num.charAt(i)) {
stack.pop();
k–;
}
stack.push(num.charAt(i));
}
while (k > 0) {
stack.pop();
k–;
}
StringBuilder sb = new StringBuilder();
while (!stack.isEmpty()) sb.append(stack.pop());
sb.reverse();
while (sb.length() > 1 && sb.charAt(0) == ‘0’) sb.deleteCharAt(0);
return sb.toString();
}
}