# 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

```java
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;
*/
```
