Leetcode: Binary Tree Level Order Traversal II
2014-10-02 13:49
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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
return its bottom-up level order traversal as:
Same as Binary Tree Level Order Traversal I, except that this time, the new level should be added to the beginning of the arraylist.
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
if (root == null) {
return res;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
while (!queue.isEmpty()) {
ArrayList<Integer> level = new ArrayList<Integer>();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
level.add(node.val);
if (node.left != null) {
queue.add(node.left);
}
if (node.right != null) {
queue.add(node.right);
}
}
res.add(0, level);
}
return res;
}
}
For example:
Given binary tree
{3,9,20,#,#,15,7},
3 / \ 9 20 / \ 15 7
return its bottom-up level order traversal as:
[ [15,7], [9,20], [3] ]
Same as Binary Tree Level Order Traversal I, except that this time, the new level should be added to the beginning of the arraylist.
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
if (root == null) {
return res;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.add(root);
while (!queue.isEmpty()) {
ArrayList<Integer> level = new ArrayList<Integer>();
int size = queue.size();
for (int i = 0; i < size; i++) {
TreeNode node = queue.poll();
level.add(node.val);
if (node.left != null) {
queue.add(node.left);
}
if (node.right != null) {
queue.add(node.right);
}
}
res.add(0, level);
}
return res;
}
}
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