1047. Remove All Adjacent Duplicates In String

You are given a string s consisting of lowercase English letters. A duplicate removal consists of choosing two adjacent and equal letters and removing them.
We repeatedly make duplicate removals on s until we no longer can.
Return the final string after all such duplicate removals have been made. It can be proven that the answer is unique.

//approach -1 (using stack)
//time complexity : O(n)   and space complexity :O(n) at worst case
class Solution {
    public String removeDuplicates(String s) {
        Stack<Character> st=new Stack<>();
        int n=s.length();
        int i=0;
        while(i<n){
            char ch=s.charAt(i);
            if(!st.isEmpty() && st.peek()==ch){
                st.pop();
            } else  {
                st.push(ch);
            }
            i++;
        }
   }
        
//approach -2 using stringbuilder method
//faster than stack approach as time complexity :O(n) and space complexity :O(1)
//here we use stringbuilder method as string is immutable .
class Solution {
    public String removeDuplicates(String s) {

        StringBuilder sb = new StringBuilder();
        int slength = 0;
        char[] arr = s.toCharArray();
        for(char car: arr){
            if(slength >0 && car == sb.charAt(slength - 1)){
                sb.deleteCharAt(slength-- - 1);
            }else{
                sb.append(car);
                slength++;
            }
        }
        return sb.toString();
    }
}