Given a positive integer n, check if it is perfect square or not using only addition/subtraction operations and in minimum time complexity.
Examples :

Input : n = 36
Output : Yes

Input : n = 2500
Output : Yes
Explanation: 2500 is a perfect square of 50

Input : n = 8
Output : No

We can use the property of odd number for this purpose: 

Addition of first n odd numbers is always perfect square 
1 + 3 = 4, 
1 + 3 + 5 = 9, 
1 + 3 + 5 + 7 + 9 + 11 = 36 ...

Below is the implementation of above idea : 

Output : 

No
Yes

How does this work?
Below is explanation of above approach. 

1 + 3 + 5 + ... (2n-1) = ∑(2*i - 1) where 1<=i<=n
 = 2*(∑(i) - ∑(1)) where 1<=i<=n
 = 2n(n+1)/2 - n
 = n(n+1) - n
 = n2

Check if given number is perfect square

Please refer the above article for all approaches and detailed explanation.