# [Question] Remove Chars in Pairs

### Question

Given a string, recursively remove adjacent duplicate characters from string. The output string should not have any adjacent duplicates.

Input: azxxzy

Output: ay

First “azxxzy” is reduced to “azzy”. The string “azzy” contains duplicates, so it is further reduced to “ay”.

### Analysis

We could do it recursively until all pairs are removed, but it’s not good.

There’s an O(n) solution.

### Solution

Most obvious solution is to use a stack. In the end, the stack stores all unmatched chars.

But we can also do it without using space (assuming the input is char array). Just use the original char array to store result, with the helper of 2 pointers. The code is very much concise.

### Code

Refactored code by me

``````public static String remove_pair(char[] input) {
int len = input.length;
int right = 1, left = 0;

while (right < len) {
// Cancel pairs
while (right < len && left >= 0 && input[right] == input[left]) {
right++;
left--;
}
if (right == len) {
break;
}
input[++left] = input[right++];
}
return String.valueOf(input).substring(0, left + 1);
}
``````