Given a binary array, find the maximum length of a contiguous subarray with equal number of 0 and 1.

Example 1:

Input: [0,1]
Output: 2
Explanation: [0, 1] is the longest contiguous subarray with equal number of 0 and 1.

Example 2:

Input: [0,1,0]
Output: 2
Explanation: [0, 1] (or [1, 0]) is a longest contiguous subarray with equal number of 0 and 1.

Note: The length of the given binary array will not exceed 50,000.

public class Solution {
    public int findMaxLength(int[] nums) {
        if(nums == null || nums.length == 0){
            return 0;
        }
        for(int i=0; i<nums.length; i++){
            if(nums[i] == 0){
                nums[i] = -1; //先把所有的0变成-1，这样就是求sum为0的最大区间
            }
        }
        int sum = 0, max = 0;
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        map.put(0, -1);
        for(int i=0; i<nums.length; i++){
            sum += nums[i];
            if(!map.containsKey(sum)){
                map.put(sum, i);
            }else{
                max = Math.max(max, i-map.get(sum));
            }
        }
        return max;
    }
}