Codeforces 627 A. XOR Equation (数学)

题目链接：http://codeforces.com/problemset/problem/627/A

题意：

        告诉你s 和 x，a + b = s    a xor b = x   a, b > 0。 让你求符合条件的a b有多少对

思路：

        a + b = s , a ^ b = x   ==>   s - x = (a & b) * 2 

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
int a[100], b[100];
int main()
{
    LL s, x;
    cin >> s >> x;
    LL y = s - x;
    if(y % 2 || x > s) {
        cout << 0 << endl;
    } else {
        y /= 2;
        int pos1 = 0, pos2 = 0, cnt = 0, fuck = y ? 1: 0;
        for( ; x; ++pos1, x >>= 1) {
            if(x & 1) {
                ++cnt;
                a[pos1] = 1;
            }
        }
        for( ; y; ++pos2, y >>= 1) {
            if(y & 1)
                b[pos2] = 1;
        }
        int ok = 1;
        for(int i = 0; i < max(pos1, pos2); ++i) {
            if(a[i] && b[i]) {
                ok = 0;
                break;
            }
        }
        if(ok) {
            if(fuck) {
                cout << (1LL << ((LL)cnt)) << endl;
            } else {
                cout << (1LL << ((LL)cnt)) - 2 << endl;
            }
        } else {
            cout << 0 << endl;
        }
    }
    return 0;
}