[Leetcode] Binary Tree Level Order Traversal
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] ]
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 stub 22 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 }