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# GATE | Gate IT 2005 | Question 50

• Difficulty Level : Medium
• Last Updated : 28 Jun, 2021

In a binary tree, for every node the difference between the number of nodes in the left and right subtrees is at most 2. If the height of the tree is h > 0, then the minimum number of nodes in the tree is:
(A) 2h – 1
(B) 2h – 1 + 1
(C) 2h – 1
(D) 2h

Explanation:

```Let there be n(h) nodes at height h.

In a perfect tree where every node has exactly
two children, except leaves, following recurrence holds.

n(h) = 2*n(h-1) + 1

In given case, the numbers of nodes are two less, therefore
n(h) = 2*n(h-1) + 1 - 2
= 2*n(h-1) - 1

Now if try all options, only option (b) satisfies above recurrence.

Let us see option (B)
n(h) = 2h - 1 + 1

So if we substitute
n(h-1) = 2h-2 + 1, we should get n(h) = 2h-1 + 1

n(h) =  2*n(h-1) - 1
=  2*(2h-2 + 1) -1
=  2h-1 + 1.
```

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