BinaryTreeLevelOrderTraversal.java

/*
Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

*/


import java.io.*;
import java.util.*;
public class BinaryTreeLevelOrderTraversal {
	public static class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;
		TreeNode(int x) { val = x; }
	}
	public static List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null) return res;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        while(!q.isEmpty()){
            int s = q.size();
            List<Integer> a = new ArrayList<Integer>();
            for(int i = 0; i < s; i++){
                TreeNode t = q.poll();
                a.add(t.val);
                if(t.left != null) q.offer(t.left);
                if(t.right!= null) q.offer(t.right);
            }
            res.add(a);
        }
        return res;
    }

	public static void main(String[] args) { 
		TreeNode root = new TreeNode(1);
		TreeNode node2 = new TreeNode(2);
		TreeNode node3 = new TreeNode(3);
		TreeNode node4 = new TreeNode(4);
		TreeNode node5 = new TreeNode(5);
		TreeNode node6 = new TreeNode(6);
		TreeNode node7 = new TreeNode(7);
		TreeNode node8 = new TreeNode(8);
		TreeNode node9 = new TreeNode(9);
		TreeNode node10 = new TreeNode(10);
		TreeNode node11 = new TreeNode(11);
		root.left = node2;
		root.right = node3;
		node2.left = node4;
		node2.right = node5;
		node3.left = node6;
		node3.right = node7;
		node4.left = node8;
		node4.right = node9;
		node5.left = node10;
		node5.right = node11;
		node6.left =  null;
		node6.right = null;
		node7.left = null;
		node7.right = null;
		node8.left = null;
		node8.right = null;
		node9.left = null;
		node9.right = null;
		node10.left = null;
		node10.right = null;
		node11.left = null;
		node11.right = null;
		List<List<Integer>> res = levelOrder(root);
		System.out.println("The level order:");
		for(int i = 0; i < res.size(); i++){
			System.out.println(res.get(i));
		}
		return;
	}	
}