367.Valid-Perfect-Square

367. Valid Perfect Square

题目地址

https://leetcode.com/problems/valid-perfect-square/

题目描述

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
Output: true

Example 2:
Input: 14
Output: false

代码

Approach #1 Binary Search

class Solution {
  public boolean isPerfectSquare(int num) {
        if (num < 2)    return true;

    long left = 2, right = num / 2;
    int x, guessSquared;
    while (left <= right) {
      x = left + (right - left) / 2;
      guessSquared = x * x;
      if (guessSquared == num) {
        return true;
      }
      if (guessSquared > num) {
        right = x - 1;
      } else {
        left = x  + 1;
      }
    }

    return false;
  }
}

Approach #2 Newton's Method

Time O(logN) Space O(1)

class Solution {
  public boolean isPerfectSquare(int num) {
    if (nums < 2) return true;

    long x = num / 2;
    while (x * x > num) {
      x = (x + num / x) / 2;
    }

    return (x * x == num);
  }
}