Introduction
In this article, we will solve the leetcode problem #1609, Even Odd Tree. The problem statement goes like this.
A binary tree is named Even-Odd if it meets the following conditions,
- The root of the binary tree is at level index
0, its children are at level index 1, their children are at level index 2, etc.
- For every even-indexed level, all nodes at the level have odd integer values in strictly increasing order (from left to right).
- For every odd-indexed level, all nodes at the level have even integer values in strictly decreasing order (from left to right).
Given the root of a binary tree, return true if the binary tree is Even-Odd, otherwise return false.
Example 1
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Input: root = [1,10,4,3,null,7,9,12,8,6,null,null,2]
Output: true
Explanation: The node values on each level are:
Level 0: [1]
Level 1: [10,4]
Level 2: [3,7,9]
Level 3: [12,8,6,2]
Since levels 0 and 2 are all odd and increasing, and levels 1 and 3 are all even and decreasing, the tree is Even-Odd.
Example 2
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Input: root = [5,4,2,3,3,7]
Output: false
Explanation: The node values on each level are:
Level 0: [5]
Level 1: [4,2]
Level 2: [3,3,7]
Node values in the level 2 must be in strictly increasing order, so the tree is not Even-Odd.
Example 3
Input: root = [5,9,1,3,5,7]
Output: false
Explanation: Node values in the level 1 should be even integers.
Example 4
Input: root = [1]
Output: true
Example 5
Input: root = [11,8,6,1,3,9,11,30,20,18,16,12,10,4,2,17]
Output: true
Solution
The below java solution uses the breadth-first search to go through all nodes layer by layer.
public class _1609 {
public static class Solution1 {
public boolean isEvenOddTree(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
int level = 0;
while (!queue.isEmpty()) {
int size = queue.size();
List<Integer> list = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
list.add(node.val);
if (node.left != null) {
queue.add(node.left);
}
if (node.right != null) {
queue.add(node.right);
}
}
if (level % 2 == 0) {
for (int i = 0; i < list.size(); i++) {
if (list.get(i) % 2 == 0) {
return false;
}
if (i < list.size() - 1 && list.get(i) >= list.get(i + 1)) {
return false;
}
}
} else {
for (int i = 0; i < list.size(); i++) {
if (list.get(i) % 2 != 0) {
return false;
}
if (i < list.size() - 1 && list.get(i) <= list.get(i + 1)) {
return false;
}
}
}
level++;
}
return true;
}
}
}