leetcode69_根号

public int mySqrt(int x) {
    int l = 0, r = x, ans = -1;
    while (l <= r) {
        int mid = l + (r-l)/2;
        if((long) mid * mid <= x) {
            ans = mid;
            l = mid +1;
        }
        else {
            r = mid - 1;
        }
    }
    return ans;
}