LeetCode 525. Contiguous Array

原题链接在这里：https://leetcode.com/problems/contiguous-array/

题目：

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.

题解：

sum = 0.

When encountering 0, sum--, when encountering 1, sum++.

Record first occurance of sum with index i. 

When same sum appread again, update res.

Time Complexity: O(n). n = nums.length.

Space: O(n).

AC Java: 

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

类似Maximum Size Subarray Sum Equals k.