leetcode402. Remove K Digits

题目要求

``````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.``````

思路和代码

``````10200 k=1

123450123 k=5

``````    public String removeKdigits(String num, int k) {
if(num == null || num.length() == 0 || k==num.length()) return "0";
Stack<Character> stack = new Stack<>();
for(int i = 0 ; i < num.length() ; i++) {
char c = num.charAt(i);
while(k> 0 && !stack.isEmpty() && stack.peek() > c) {
stack.pop();
k--;
}
stack.push(c);
}

while(k > 0) {
stack.pop();
k--;
}

StringBuilder result = new StringBuilder();
while(!stack.isEmpty()) {
result.append(stack.pop());
}
while(result.length() > 1 && result.charAt(result.length()-1) == '0') {
result.deleteCharAt(result.length()-1);
}
return result.reverse().toString();
}``````