525. Contiguous Array

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.

Solution:

1. Replace all 0s to -1s in the array.

2. Problem becomes find maximum size subarray sum equals 0. (Maximum Size Subarray Sum Equals k)



Code:

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