Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 4 | Trees and Graphs
4.4 Given a binary search tree, design an algorithm which creates a linked list of all the
nodes at each depth (eg, if you have a tree with depth D, you’ll have D linked lists).
SOLUTION
pg 54
We can do a simple level by level traversal of the tree, with a slight modification of the breathfirst
traversal of the tree.
In a usual breath first search traversal, we simply traverse the nodes without caring which
level we are on. In this case, it is critical to know the level. We thus use a dummy node to
indicate when we have finished one level and are starting on the next.
1 ArrayList findLevelLinkList(TreeNode root) {
2 int level = 0;
3 ArrayList result =
4 new ArrayList();
5 LinkedList list = new LinkedList();
6 list.add(root);
7 result.add(level, list);
8 while (true) {
9 list = new LinkedList();
10 for (int i = 0; i < result.get(level).size(); i++) {
11 TreeNode n = (TreeNode) result.get(level).get(i);
12 if (n != null) {
13 if(n.left != null) list.add(n.left);
14 if(n.right!= null) list.add(n.right);
15 }
16 }
17 if (list.size() > 0) {
18 result.add(level + 1, list);
19 } else {
20 break;
21 }
22 level++;
23 }
24 return result;
25 }
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