LeetCode OJ：Binary Tree Level Order Traversal（二叉树的层序遍历）

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]
]

层序遍历，既可以用dfs实现也可以用bfs实现，用bfs实现的时候应该借助队列，代码如下：

dfs:

 class Solution {
 public:
     vector<vector<int>> levelOrder(TreeNode* root) {
         dfs(root, );
         return ret;
     }
     void dfs(TreeNode * root, int dep)
     {
         if(!root) return;
         if(dep < ret.size()){
             ret[dep].push_back(root->val);
         }else{
             vector<int> tmp;
             tmp.push_back(root->val);
             ret.push_back(tmp);
         }
         dfs(root->left, dep+);
         dfs(root->right, dep+);
     }
 private:
     vector<vector<int>> ret;
 };

bfs:

 class Solution {
 private:
     struct Node{
         TreeNode * treeNode;
         int level;
         Node(){}
         Node(TreeNode * node, int lv)
         :treeNode(node), level(lv){}
     };
 public:
     vector<vector<int>> levelOrder(TreeNode* root) {
         queue<Node> nodeQueue;
         vector<vector<int>> ret;
         if(root == NULL)
             return ret;
         nodeQueue.push(Node(root, ));
         int dep = -;
         while(!nodeQueue.empty()){
             Node node = nodeQueue.front();
             if(node.treeNode->left)
                 nodeQueue.push(Node(node.treeNode->left, node.level + ));
             if(node.treeNode->right)
                 nodeQueue.push(Node(node.treeNode->right, node.level + ));
             if(dep == node.level)
                 ret[dep].push_back(node.treeNode->val);
             else{
                 vector<int> tmp;
                 dep++;
                 ret.push_back(tmp);
                 ret[dep].push_back(node.treeNode->val);
             }
             nodeQueue.pop();
         }
         return ret;
     }
 };

java版本的如下所示，同样的包括dfs以及bfs：

 public class Solution {
     public List<List<Integer>> levelOrder(TreeNode root) {
         List<List<Integer>> ret = new ArrayList<List<Integer>>();
         dfs(ret, root);
         return ret;
     }
 
     public void dfs(List<List<Integer>> ret, int dep, TreeNode root){
         if(root == null)
             return;
         if(dep < ret.size()){
             ret.get(i).add(root.val);
         }else{
             List<Integer> tmp = new ArrayList<Integer>();
             tmp.add(root.val);
             ret.add(tmp);
         }
         dfs(ret, dep+1, root.left);
         dfs(ret, dep+1, root.right);
     }
 }

下面的是bfs，稍微麻烦一点，需要自己再创造一个数据结构，代码如下：

 public class Solution {
     public class Node{
         int level;
         TreeNode node;
         Node(TreeNode n, int l){
             level = l;
             node = n;
         }
     }
     public List<List<Integer>> levelOrder(TreeNode root) {
         int dep = -1;
         List<List<Integer>> ret = new ArrayList<List<Integer>>();
         Queue<Node> queue = new LinkedList<Node>();
         queue.add(new Node(root, 0));
         while(!queue.isEmpty()){
             Node n = queue.poll();
             if(n.node == null) continue;
             if(dep < n.level){
                 List<Integer> tmp = new ArrayList<Integer>();
                 tmp.add(n.node.val);
                 ret.add(tmp);
                 dep++;
             }else{
                 ret.get(dep).add(n.node.val);
             }
             if(n.node.left != null) queue.add(new Node(n.node.left, n.level + 1));
             if(n.node.right != null) queue.add(new Node(n.node.right, n.level + 1));
         }
         return ret;
     }
 }