LeetCode.525 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.

分析：

class Solution {
    public int findMaxLength(int[] nums) {
        //给定数组，找出子串中0和1个数相等的最长的子串长度
        //思路：该子串一定是由成对的0和1组成的，将其中0全部变为-1（精妙之处）。类似求最长子串的最优解解法
        //再用HashMap存储sum，如果再次出现相同的sum，说明期间0和1保持平衡
        if(nums.length==0||nums==null) return 0;
        
        for(int i=0;i<nums.length;i++){
            if(nums[i]==0){
                nums[i]=-1;
            }
        }
        
        HashMap<Integer,Integer> hm=new HashMap<Integer,Integer>();
        int sum=0,max=0;
        //初始化第一个和->长度
        hm.put(0,-1);
        for(int i=0;i<nums.length;i++){
            sum+=nums[i];
            if(hm.containsKey(sum)){
                //hm存入长度的和->下标
                max=Math.max(max,i-hm.get(sum));
            }else{
                hm.put(sum,i);
            }
        }
        return max;
    }
}