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.
题目含义：给定一个只含有0,1的整数数组，找到数组中最长的一个子序列，该序列中1的个数和0的个数相同。

 1     public int findMaxLength(int[] nums) {
 2 //        这里有一个把0转换成-1来进行计算的过程，然后存储一个hashmap用来存放sum，如果接下来的sum在hashmap里面出现过，
 3 // 则说明hashmap里面出现的那个值的索引的开始（不包括索引），到目前的索引的和为0
 4         for (int i = 0; i < nums.length; i++) {
 5             if (nums[i] == 0) nums[i] = -1;
 6         }
 7 
 8         Map<Integer, Integer> sumToIndex = new HashMap<>();
 9         sumToIndex.put(0, -1);
10         int sum = 0, max = 0;
11 
12         for (int i = 0; i < nums.length; i++) {
13             sum += nums[i];
14             if (sumToIndex.containsKey(sum)) {
15                 max = Math.max(max, i - sumToIndex.get(sum));
16             }
17             else {
18                 sumToIndex.put(sum, i);
19             }
20         }
21 
22         return max;        
23     }