LintCode Sqrt(x)

Implement int sqrt(int x).
Compute and return the square root of x.
Have you met this question in a real interview? Yes
Example
sqrt(3) = 1
sqrt(4) = 2
sqrt(5) = 2
sqrt(10) = 3
Challenge
O(log(x))
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class Solution {
public:
    /**
     * @param x: An integer
     * @return: The sqrt of x
     */
    int sqrt(int x) {
        // write your code here
        if (x < 1) {
            return 0;
        }
        long lo = 1;
        long hi = x / 2 + 1;
        while (lo < hi) {
            long mid = (lo + hi) / 2;
            long prod = mid * mid;
            if (prod <= x) {
                lo = mid + 1;
            } else {
                hi = mid;
            }
        }
        
        return lo - 1;
    }
};

还是依照upper_bound的思路