二分查找 – 有序序列猜得快而准

int binarySearch(vector<int> a, int v) {
    int i = 0, j = a.size() - 1;
    while(i < j) {
        int m = (i + j) / 2;
        if(a[m] == v) return m;
        if(a[m] < v) j = m - 1;
        else i = m + 1;
    }
    return -1;
}