Single Number II

• Given an array of integers, every element appears three times except for one. Find that single one.

• Note:
  Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

  目前的算法是基于map的，leetcode里面不能调用c++ STL中的hash_map函数。这道题还有更好的方法，不占用额外的内存，暂时给出占用额外内存的方法:).C++代码如下：

class Solution {
public:
    int singleNumber(int A[], int n) {
    map<int, int> countMap;
    int num = 0;
    if (n <= 0)
        return -1;
    for (int i = 0; i< n; i++)
    {
        if (countMap.find(A[i]) != countMap.end())
            countMap[A[i]] += 1;
        else
            countMap[A[i]] = 1;;
    }

    for (int i = 0; i < n; i++)
    {
        if (countMap[A[i]] == 1)
            return A[i];
    }
    }
};