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.

Algorithm:

1. Initialize two variables `curSum` and `max`

2. Loop through the array and increment `curSum` by 1 if nums[i] is 1 else decrement 1

3. Now you can encounter two cases

1. Sub array formed from index 0

In this case just take the current index and add 1

In above examples it would be 1+1 = 2 for 1st example and 3+1 = 4 for 2nd example

2. Sub Array formed after index 0

In this scenario you have to find the previous index of cursum and subtract it

In above image it would be 5-1 = 4

Here’s the Code:

``````/**
* @param {number[]} nums
* @return {number}
*/
var findMaxLength = function(nums) {
let curSum = 0, max =0;
let map = new Map();

for(let i = 0; i < nums.length; i++) {
if(nums[i] == 0) {
curSum--;
} else {
curSum++;
}

if(curSum == 0) {
max = i + 1
}

if(map.has(curSum)) {
max = Math.max(max, i - map.get(curSum))
} else {
map.set(curSum, i)
}
}

return max;
};
``````

You can optimize the if condition where curSum == 0 by adding a value into map {0: -1}

Now, in the condition map.has(curSum), it will be something like 1 - (-1) = 2

This post is also available on DEV.