LeetCode OJ：Binary Tree Preorder Traversal（前序遍历二叉树）

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3}

 1
    \
     2
    /
   3

return [1,2,3].

前序遍历二叉树，只不过题目的要求是尽量不要使用递归，当然我还是先用递归写了一个：

 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode* root) {
         ret.clear();
         if(root != NULL)
             preTrans(root);
         return ret;
     }
     void preTrans(TreeNode * root){
         ret.push_back(root->val);
         if(root->left != NULL) preTrans(root->left);
         if(root->right != NULL) preTrans(root->right);
     }
 private:
     vector<int> ret;
 };

当然还有非递归的方法，递归那么当然要使用到stack，其实这种方法写起来有点像是java中的递归的迭代器：

 class Solution {
 public:
     vector<int> preorderTraversal(TreeNode* root) {
         vector<int> ret;
         if(root == NULL) return ret;
         stack<TreeNode*> treeStk;
         treeStk.push(root);
         TreeNode * tmpNode;
         while(!treeStk.empty()){
             tmpNode = treeStk.top();
             treeStk.pop();
             ret.push_back(tmpNode->val);
             if(tmpNode->left != NULL) treeStk.push(tmpNode->left);
             if(tmpNode->right != NULL) treeStk.push(tmpNode->right);
         }
         return ret;
     }
 };

java版本的代码如下所示，首先是递归：

 public class Solution {
     public List<Integer> preorderTraversal(TreeNode root) {
         List<Integer> ret = new ArrayList<Integer>();
         tranversal(root, ret);
         return ret;
     }

     public void tranversal(TreeNode root, List<Integer> ret){
         if(root != null){
             ret.add(root.val);
             tranversal(root.left, ret);
             tranversal(root.right, ret);
         }
         return;
     }
 }

然后是非递归的方式：

 public class Solution {
     public List<Integer> preorderTraversal(TreeNode root) {
         Stack<TreeNode> s = new Stack<TreeNode>();
         List<Integer> ret = new ArrayList<Integer>();
         s.push(root);
         while(!s.isEmpty()){
             TreeNode t = s.pop();
             if(t == null)
                 continue;
             ret.add(t.val);
             s.push(t.right); //注意因为使用的是栈，所以应该先push right.
             s.push(t.left);
         }
         return ret;
     }
 }