leetcode 刷題之路 79 Binary Tree Preorder Traversal

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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?

非遞迴實現樹的前序走訪。前序走訪比較簡單，注意入棧的時候應該按照右孩子節點在前的順序，這樣才能保證出棧順序左先右後。

代碼如下：

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