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:
1 class Solution { 2 public int findMaxLength(int[] nums) { 3 if(nums == null || nums.length == 0){ 4 return 0; 5 } 6 7 HashMap<Integer, Integer> hm = new HashMap<>(); 8 hm.put(0, -1); 9 int sum = 0; 10 int res = 0; 11 12 for(int i = 0; i<nums.length; i++){ 13 sum += nums[i] == 0 ? -1 : 1; 14 if(hm.containsKey(sum)){ 15 res = Math.max(res, i - hm.get(sum)); 16 }else{ 17 hm.put(sum, i); 18 } 19 } 20 21 return res; 22 } 23 }