[Leetcode] binary tree level order traversal

Given a binary tree, returnLevel orderTraversal 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:

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

Solution 1: BFS

 1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public List<List<Integer>> levelOrder(TreeNode root) {12         List<List<Integer>> ret=new ArrayList<List<Integer>>();13         if(root==null)14             return ret;15         16         Queue<TreeNode> queue=new LinkedList<TreeNode>();17         List<Integer> al=new ArrayList<Integer>();18         19         queue.add(root);20         int curLvl=1;21         int nexLvl=0;22         23         while(!queue.isEmpty()){24             TreeNode cur=queue.remove();25             al.add(cur.val);26             curLvl--;27             28             if(cur.left!=null){29                 queue.add(cur.left);30                 nexLvl++;31             }32             if(cur.right!=null){33                 queue.add(cur.right);34                 nexLvl++;35             }36             if(curLvl==0){37                 ret.add(al);38                 al=new ArrayList<Integer>();39                 curLvl=nexLvl;40                 nexLvl=0;41             }42         }43         44         return ret;45     }46 }

 

Solution 2: DFS

 1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     12     List<List<Integer>> ret;13     14     public List<List<Integer>> levelOrder(TreeNode root) {15         ret = new ArrayList<List<Integer>>();16         goDeeper(0, root);17         return ret;18     }19 20     private void goDeeper(int level, TreeNode root) {21         // TODO Auto-generated method stub22         if (root == null)23             return;24         if (ret.size() > level) {25             List<Integer> a = ret.get(level);26             a.add(root.val);27         } else {28             List<Integer> a = new LinkedList<Integer>();29             a.add(root.val);30             ret.add(a);31         }32         goDeeper(level+1, root.left);33         goDeeper(level+1, root.right);34     }35 }

 

[Leetcode] binary tree level order traversal