leetcode Question: Binary Tree Preorder Traversal (iteration)

Binary Tree Preorder Traversal (iteration)

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].

Note: Recursive solution is trivial, could you do it iteratively?

Analysis:

Classical problem,  the recursion version of the solution can be found here (you need do slight modification, can you do that?) . In this post, I solved it iteratively.  Which data structure do remind us ?  Yes! Stack!

The algorithm has following steps:
(1) Push the root node into the stack.
(2) while the stack is not empty, do:
       a. pop the top node and print it.
       b. push the right child of the top node (if exists)
       c. push the left child of the top node (if exists)

Code(C++):

/**
/**
 * Definition for binary tree
 * 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) {
        stack<TreeNode*> st;
        vector<int> res;
        if (!root){return res;}
        st.push(root);
        TreeNode* head=root;
        while (!st.empty()){
            TreeNode* top = st.top();
            res.push_back(top->val);
            st.pop();
            if (top->right!=NULL){st.push(top->right);}
            if (top->left!=NULL){st.push(top->left);}
        }
        return res;
    }
};

Code(Python):

# Definition for a  binary tree node
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a list of integers
    def preorderTraversal(self, root):
        res = [] 
        st = []
        if root != None:
            st.append(root)
            while len(st) != 0:
                tmp = st.pop()
                res.append(tmp.val)
                if (tmp.right != None):
                    st.append(tmp.right)
                if (tmp.left != None):
                    st.append(tmp.left)
        return res