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.

class Solution {
    public int findMaxLength(int[] nums) {
            Map<Integer, Integer> map = new HashMap<>();
            int maxlen = 0, count = 0;
            for(int i = 0;i < nums.length;i++) {
                count += (nums[i] == 1)?1:-1;
                    // Array from index 0 to i contains equal number of 0's and 1's
                if(count == 0)
                    maxlen = Math.max(maxlen, i+1);            
                if(map.containsKey(count)) {
                    maxlen = Math.max(maxlen, i - map.get(count));
                } else 
                    map.put(count, i);
            }
            return maxlen;
        }
}

 设置一个map存放count，index的pair。计算count，nums是1就+1，是0就-1，然后有三种情况：count==0说明此时满足1==0数量，更新maxlen；map包含了count，那就用当前的index减去上一个count的index；不包含count，put进map