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;
    }
}