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.

/**
 * 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 equal 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 of sum -> index
        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;
    }
}