844. Backspace String Compare

Given two strings S and T, return if they are equal when both are typed into empty text editors. # means a backspace character.
Example 1:
Input: S = "ab#c", T = "ad#c"
Output: true
Explanation: Both S and T become "ac".
Example 2:
Input: S = "ab##", T = "c#d#"
Output: true
Explanation: Both S and T become "".
Example 3:
Input: S = "a##c", T = "#a#c"
Output: true
Explanation: Both S and T become "c".
Example 4:
Input: S = "a#c", T = "b"
Output: false
Explanation: S becomes "c" while T becomes "b".





class Solution {
    public boolean backspaceCompare(String S, String T) {
        return simplify(S).equals(simplify(T));
    
    }
    private String simplify(String s){
        StringBuilder sb = new StringBuilder();
        Stack<Character> stack = new Stack<>();
        for(int i = 0; i < s.length(); i++){
            char c = s.charAt(i);
            if(c == '#' && !stack.isEmpty()){
                stack.pop();
            }
            if(Character.isLetter(c)){ // don't use else here. incase when the stack is empty and cur char is "#"
                stack.push(c);
            }
        }
        while(!stack.isEmpty()) sb.append(stack.pop());
        return sb.toString(); // need to convert to string . can't use .equals for stringBuilder ? 
    }
}



Two StringBuilder objects are never equal. Use .toString() to get the string representation for both the objects and then use .equals() to compare the objects.