Amazon Interview Experience | Set 356 (For 6 Year Experienced)

I went through some interview rounds for SDE-2 in amazon. Below is the format.
Round1: It was a test on HackerEarth with two programming questions

• HackerEarth test link

Round 2:

1. Boundary Traversal of binary tree
   w3wiki Link
2. Find four elements a, b, c and d in an array such that a+b = c+d
   w3wiki Link

Write production ready code. Consider scenario for duplicates too.

Round 3:

1. Trapping Rain Water
   w3wiki Link
2. Next smallest palindrome
   w3wiki Link