09. Valid_Perfect_Square.java

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

    long left = 2, right = num / 2, x, guessSquared;
    while (left <= right) {
      x = left + (right - left) / 2;    //midpoint
      guessSquared = x * x;   //guess squared sort of binary search
      if (guessSquared == num) {
        return true;
      }
      if (guessSquared > num) {
        right = x - 1;
      } else {
        left = x + 1;
      }
    }
    return false;
  }
}