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Single_Number.cpp

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class Solution {

public:

int singleNumber(int A[], int n) {

// A XOR A = 0, and A XOR B XOR A = B

int res = A[0];

for (int i = 1; i < n; i++) {

res = res ^ A[i];

}

return res;

/*

sort(A, A + n);

for(int i = 0; i < n - 1; i += 2) {

if(A[i] != A[i + 1]) return A[i];

}

return A[n - 1];

*/

// extra space

/*

map <int, int> freq;

for(int i = 0; i < n; ++i) ++freq[A[i]];

for(int i = 0; i < n; ++i) if(freq[A[i]] < 2) return A[i];

*/

}

};