[LeetCode] 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.

Thought process:
Iterate through the array. Use a variable count to keep track of 0s and 1s. If I see a 1, increment count; Else, decrement count. The trick is that when I see two indices with the same count, the sub-array between them must have equal number of 0 and 1. Use a hash map of count -> index to keep track of this information.

Solution:

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

Time complexity: O(n).
Location: Philadelphia, PA, USA

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