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
解题方法，用二分法。因为说明了是正数，所以num>=1。
func isPerfectSquare(num int) bool {
if num == 1 {
return true
}
left, right := 0, num/2
for left <= right {
mid := (left + right) / 2
val := mid * mid
if val == num {
return true
} else if val > num {
right = mid - 1
} else {
left = mid + 1
}
}
return false
}
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