openjudge 01:查找最接近的元素 入门二分

openjudge 01:查找最接近的元素
快NOIP了，可是还是好弱。
那……来一场愉快的二分之旅吧！

#include 
#include 
#include 
#include 
#include 
using namespace std;

#define MAXN (100100)
int num[MAXN];

int main()
{
    int n;
    cin >> n;
    for(int i = 1; i <= n; ++ i) scanf("%d", &num[i]);
    int m;
    cin >> m;
    while(m --)
    {
        int x, ans;
        cin >> x;
        int l = 1, r = n, mid;
        if(x <= num[1]) {cout << num[1] << endl; continue;}
        if(x >= num[n]) {cout << num[n] << endl; continue;}
        while(l <= r)
        {
            mid = (l+r)>>1;
            if(num[mid] <= x)
                l = mid+1;
            else r = mid-1;
        }
        // r <= l
        if(abs(num[l]-x) < abs(num[r]-x))
            ans = l;
        else ans = r;
        cout << num[ans] << endl;
    }
    return 0;
}

/*
10
0 1 2 5 6 7 11 13 19 20
-1
*/