題目

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.

題意

判斷給定值是否為完全平方數

Example 1:

Input: num = 16
Output: true

Example 2:

Input: num = 14
Output: false

解題想法

宣告最小數min = 1，最大數max = Math.floor(num / 2)，
使用二分法縮小範圍，取得中間值mid = (min + max) / 2，
若mid的平方小於參數，將min設為mid，
若mid的平方大於參數，將max設為mid。

Solution

var isPerfectSquare = function (num) {
  if (num < 1) return false;
  if (num === 1) return true;
  let min = 1;
  let max = Math.floor(num / 2);

  while (min <= max) {
    let mid = (min + max) / 2;
    if (mid * mid == num) {
      return true;
    } else if (mid * mid < num) min = Math.floor(mid + 1);
    else {
      max = Math.ceil(mid - 1);
    }
  }
  return false
};