LeetCode-N-Repeated Element in Size 2N Array

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Description：
In a array A of size 2N, there are N+1 unique elements, and exactly one of these elements is repeated N times.

Return the element repeated N times.

Example 1:

Input: [1,2,3,3]
Output: 3

Example 2:

Input: [2,1,2,5,3,2]
Output: 2

Example 3:

Input: [5,1,5,2,5,3,5,4]
Output: 5

Note:

• 4 <= A.length <= 10000
• 0 <= A[i] < 10000
• A.length is even

题意：给定一个包含2N个元素的一维数组，找出数组中唯一出现的出现次数为N次的那个元素；

解法：我们需要统计哪个元素的次数为N次，可以考虑利用哈希表来实现，这样，我们在遍历数组时，记录元素出现的次数，当此元素出现的次数已经为N时，那么这个元素就是最终的答案（因为数组中出现次数为N的元素是唯一的）；

Java

class Solution {
    public int repeatedNTimes(int[] A) {
        Map<Integer, Integer> frequency = new HashMap<>();
        int res = A[0];
        for (int x: A) {
            frequency.put(x, frequency.getOrDefault(x, 0) + 1);
            if (frequency.get(x) == A.length / 2) {
                res = x;
                break;
            }
        }
        
        return res;
    }
}