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.

316. Remove Duplicate Letters

321. Create Maximum Number

402. Remove K Digits

这三道题都用到了 stack 来求原序列中不打乱相对次序的最小子序列的技巧: 

while(k>0 && !stack.isEmpty() && stack.peek()>num.charAt(i)) 栈内元素是由大到小排列的, 站内元素也是需要去除的,
保证最高位的元素最小.所以遍历时碰到比栈内小的元素就pop

public class Solution {
    public String removeKdigits(String num, int k) {
        int len = num.length();
        //corner case
        if(k==len)        
            return "0";
            
        Stack<Character> stack = new Stack<>();
        int i =0;
        while(i<num.length()){
            //whenever meet a digit which is less than the previous digit, discard the previous one
            while(k>0 && !stack.isEmpty() && stack.peek()>num.charAt(i)){
                stack.pop();
                k--;
            }
            stack.push(num.charAt(i));
            i++;
        }
        
        // corner case like "1111"
        while(k>0){
            stack.pop();
            k--;            
        }
        
        //construct the number from the stack
        StringBuilder sb = new StringBuilder();
        while(!stack.isEmpty())
            sb.append(stack.pop());
        sb.reverse();
        
        //remove all the 0 at the head
        while(sb.length()>1 && sb.charAt(0)=='0')
            sb.deleteCharAt(0);
        return sb.toString();
    }
}