367. Valid Perfect Square

1. Question

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

Follow up: Do not use any built-in library function such as sqrt.

2. Examples

Example 1:

Input: num = 16
Output: true

Example 2:

Input: num = 14
Output: false

3. Constraints

• 1 <= num <= 2^31 - 1

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/valid-perfect-square 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

5.1. 使用工具类

class Solution {
  public boolean isPerfectSquare(int num) {
    int k = (int)Math.sqrt(num);
    return k * k == num;
  }
}

5.2. 二分查找

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