The idea is to change `0`

in the original array to `-1`

. Thus, if we find `SUM[i, j] == 0`

then we know there are even number of `-1`

and `1`

between index `i`

and `j`

. Also put the `sum`

to `index`

mapping to a HashMap to make search faster.

```
public class Solution {
public int findMaxLength(int[] nums) {
for (int i = 0; i < nums.length; i++) {
if (nums[i] == 0) nums[i] = -1;
}
Map<Integer, Integer> sumToIndex = new HashMap<>();
sumToIndex.put(0, -1);
int sum = 0, max = 0;
for (int i = 0; i < nums.length; i++) {
sum += nums[i];
if (sumToIndex.containsKey(sum)) {
max = Math.max(max, i - sumToIndex.get(sum));
}
else {
sumToIndex.put(sum, i);
}
}
return max;
}
}
```