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Level_Order.java

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package Algorithms;

import java.util.ArrayDeque;

import java.util.ArrayList;

import java.util.Queue;

public class Level_Order {

public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {

ArrayList<ArrayList<Integer>> levelOrder = new ArrayList<ArrayList<Integer>>();

if (root == null){

return levelOrder;

}

// record the level of the tree. 0: root.

//int level = 0;

Queue<TreeNode> queue = new ArrayDeque<TreeNode>();

// add the root to the queue.

queue.add(root);

// init the 0 level of the order.

//levelOrder[level] = new ArrayList<Integer>;

// get all the elements out from the queue.

while(!queue.isEmpty()){

// get the size of this level;

int size = queue.size();

ArrayList<Integer> tmpList = new ArrayList<Integer>();

levelOrder.add(tmpList);

// add this level to the array and store all the elements in the next level.

for(int i = 0; i < size; i++){

TreeNode currElement = queue.remove();

// add all the element in this level to the arraylist.

tmpList.add(currElement.val);

// add all the element in the next level to the queue.

if(currElement.left != null){

queue.add(currElement.left);

}

if(currElement.right != null){

queue.add(currElement.right);

}

//level ++;

}

return levelOrder;

}