Easy
有一个不用转换类型的方法，写mid*mid == x时换成mid == x/mid或者mid < x/mid这样,可以防止越界

class Solution {
    public int mySqrt(int x) {
        if (x == 0){
            return 0;
        }
        int start = 1;
        int end = x;
        int mid = 0;
        while (start + 1 < end){
            mid = start + (end - start) / 2;
            if (mid == x / mid){
                return mid;
            } else if (mid < x / mid){
                start = mid;
            } else {
                end = mid;
            }
        }
        return start;
    }
}