用類似動態規劃的思路來解決。

用HashMap是因為HashMap查詢key更快。

public class Solution {
    public int findMaxLength(int[] nums) {
        // change 0 to -1
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == 0) nums[i] = -1; 
        }
        // key: sum from 0, value: index
        Map<Integer, Integer> sumMap = new HashMap<>();
        int max = 0;
        int sum = 0;
        sumMap.put(0, -1);
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (sumMap.containsKey(sum)) {
                max = Math.max(max, i - sumMap.get(sum));
            }
            else {
                sumMap.put(sum, i);
            }
        }
        return max;
    }
}