Given a stringSof lowercase letters, aduplicate removalconsists of choosing two adjacent and equal letters, and removingthem.

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 is guaranteed the answer is unique.

Example 1:

Input: "abbaca"
Output: "ca"
Explanation: 
For example, in "abbaca" we could remove "bb" since the letters are adjacent and equal, and this is the only possible move. The result of this move is that the string is "aaca", of which only "aa" is possible, so the final string is "ca".

Note:

1. 1 <= S.length <= 20000
2. Sconsists only of English lowercase letters.

class Solution {
    public String removeDuplicates(String S) {
        char[] ch = S.toCharArray();
        Stack<Character> stack = new Stack<>();
        for(int i = 0; i < ch.length; i++){
            if(!stack.isEmpty()){
                
 
if(ch[i] == stack.peek()) stack.pop();
                else stack.push(ch[i]);
            }
            else stack.push(ch[i]);
        }
        StringBuilder res = new StringBuilder();
        while(!stack.isEmpty()) {
            res.append(stack.pop());
        }
        return res.reverse().toString();
    }
}

Stack啊Stack，FILO，空就push，不空就和當前char做對比，如果相同就pop，不同就push