Create 0367 Valid Perfect Square.md

Author: jjjllxx

math/0367 Valid Perfect Square.md

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+# 367. Valid Perfect Square
+https://leetcode-cn.com/problems/valid-perfect-square/  
+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.   
+
+
+Example 1:  
+Input: num = 16  
+Output: true  
+
+Example 2:   
+Input: num = 14  
+Output: false  
+
+Constraints:  
+1 <= num <= 2^31 - 1  
+
+``` python3
+class Solution:
+    def isPerfectSquare(self, num: int) -> bool:
+        x0=num
+        while True:
+            x1=0.5*(x0+num/x0)
+            if x0-x1<1e-7:
+                break
+            x0=x1
+        return int(x0)*int(x0)==num 
+```