69. Sqrt(x)

Easy

Binary Search O(logN), 总结点小经验，碰到会一直求平方的情况，一开始用long定义变量可以防止中间溢出.

class Solution {
    public int mySqrt(int x) {
        if (x == 0){
            return 0;
        }
        //8
        //start 2
        //end 3 
        //mid 3
        //square 9
        long start = 1;
        long end = x;
        long mid = 0;
        while (start + 1 < end){
            mid = start + (end - start) / 2;
            long square = mid*mid;
            if (square == x){
                return (int) mid;
            } else if (square > x){
                end = mid;
            } else {
                start = mid;
            }
        }
        return (int) start;
        
    }
}

Naive时间复杂度O(N)

class Solution {
    public int mySqrt(int x) {
        if (x == 0){
            return 0;
        }
        long i = 1;
        while (i*i <= x){
            if (i*i == x){
                return (int) i;
            }
            i++;
        }
        return (int)(i - 1);
    }
}