402.RemoveKDigits.java

class Solution {
    public String removeKdigits(String num, int k) {
        Stack<Character> stack = new Stack<Character>();
        
        int i = 0;
        
        while (i < num.length()) {
            
            // evict bigger digits in the beginning
            while (k > 0 && !stack.isEmpty() && stack.peek() > num.charAt(i)) {
                stack.pop();
                k--;
            }
            
            stack.push(num.charAt(i++));
        }
        
        // corner cases like 1234 or 1111
        while (k > 0) {
            stack.pop();
            k--;
        }
        
        StringBuilder sb = new StringBuilder();
        
        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }
        
        //reverse stack order
        sb.reverse();
        
        StringBuilder ans = new StringBuilder();
        
        i = 0;
        
        // evict leading 0's
        while (i < sb.length() && sb.charAt(i) == '0') {
            i++;
        }
        
        while (i < sb.length()) {
            ans.append(sb.charAt(i++));
        }
        
        // return 0 for empty string
        return ans.length() > 0 ? ans.toString() : "0";
    }
}