525. Contiguous Array

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Given a binary array, find the maximum length of a contiguous subarray with equal number of 0 and 1.

Example 1:

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Input: [0,1]
Output: 2
Explanation: [0, 1] is the longest contiguous subarray with equal number of 0 and 1.

Example 2:

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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.

解法1：

很好的一题，用到了2sum = zero的特殊解法（用hashmap）， 也用到了变换条件使得题目变得更有利。
把0变成-1的话这题的条件就变成找一个continuous array使得加和为1并且最长。
加和为0很好办，看sum是否出现过，如果出现过，更新最长距离。要注意初始条件，第一个点是设成map.put(0, -1)
C++

1

Java

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public class Solution {
    public int findMaxLength(int[] nums) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == 0) {
                nums[i] = -1;
            }
        }
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        map.put(0, -1);
        int sum = 0, max = 0;
        for (int i = 0; i < nums.length; i++) {
            sum += nums[i];
            if (map.containsKey(sum)) {
                // prefix sum exists, meaning sum[i, j] = 0 => equal number of 1 and 0 (now -1)
                max = Math.max(max, i - map.get(sum));
            } else {
                map.put(sum, i);
            }
        }
        return max;
    }
}