367. Valid Perfect Square

Given a positive integer num, write a function which returns True if num is a perfect square else False.

Note: Do not use any built-in library function such as 

sqrt

.

Example 1:

Input: 16
Returns: True

Example 2:

Input: 14
Returns: False

题意：判断一个数是否是完全平方数。
思路：二分查找。注意int数范围越界问题。

class Solution {
public:
bool isPerfectSquare(int num) {
if (num == 1)
return true;
int left = 1;
int right = num >> 1;
while (left <= right){
long long int mid = (left + right) / 2;
long long int  n = mid*mid;
if (n == num){
return true;
}
else if (n > num){
right = mid - 1;
}
else{
left = mid + 1;
}
}
return false;
}
};

更多思路，参考http://www.cnblogs.com/guoguolan/p/5619443.html