0525. Contiguous Array

https://leetcode.com/problems/contiguous-array

Description

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

Example 1:

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

Example 2:

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

Constraints:

• 1 <= nums.length <= 105

• nums[i] is either 0 or 1.

ac

class Solution {
    public int findMaxLength(int[] nums) {
        // edge cases
        if (nums == null || nums.length == 0) return 0;

        Map<Integer, Integer> map = new HashMap<>();
        map.put(0, -1);

        int sum = 0, max = 0;
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i] == 1 ? 1 : -1;

            if (map.containsKey(sum)) {
                max = Math.max(max, i - map.get(sum));
            } else {
                map.put(sum, i);
            }
        }

        return max;
    }
}

/*
1) maintain sum, meet 1 +1, meet 0 -1; 2) if current sum appear before, then the gap is the subarray we want, calculate the length; 3) put new sum-value in map;
*/