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BottomsUpLevelOrderTraversalBinaryTree.java
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BottomsUpLevelOrderTraversalBinaryTree.java
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package Leetcode;
import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;
/**
* @author kalpak
*
* Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
*
* For example:
* Given binary tree [3,9,20,null,null,15,7],
* 3
* / \
* 9 20
* / \
* 15 7
* return its bottom-up level order traversal as:
* [
* [15,7],
* [9,20],
* [3]
* ]
*
*/
public class BottomsUpLevelOrderTraversalBinaryTree {
public static List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> result = new ArrayList<>();
Deque<TreeNode> queue = new LinkedList<>();
if(root == null)
return result;
queue.add(root);
while(!queue.isEmpty()) {
int len = queue.size();
List<Integer> temp = new ArrayList<>();
while(len > 0) {
TreeNode current = queue.poll();
temp.add(current.val);
if(current.left != null)
queue.offer(current.left);
if(current.right != null)
queue.offer(current.right);
len--;
}
result.add(0, temp);
}
return result;
}
public static void main(String[] args) {
TreeNode root = new TreeNode(12);
root.left = new TreeNode(7);
root.right = new TreeNode(1);
root.left.left = new TreeNode(9);
root.right.left = new TreeNode(10);
root.right.right = new TreeNode(5);
List<List<Integer>> result = levelOrderBottom(root);
System.out.println("Reverse level order traversal: " + result);
}
}