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。

    public int findMaxLength(int[] nums) {
//        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，
// 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
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == 0) nums[i] = -1;
        }

        Map<Integer, Integer> sumToIndex = new HashMap<>();
        sumToIndex.put(0, -1);
        int sum = 0, max = 0;

        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (sumToIndex.containsKey(sum)) {
                max = Math.max(max, i - sumToIndex.get(sum));
            }
            else {
                sumToIndex.put(sum, i);
            }
        }

        return max;        
    }