leetcode 第94題 Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.

For example:

Given binary tree {1,#,2,3},

1

\

2

/

3

return [1,3,2].

思路1：遞歸實作。

C++實作：

/**
 * 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> inorderTraversal(TreeNode* root) {
       ivec.clear();
       inorder(root);
       return ivec;
    }
private:
    vector<int> ivec;
    void inorder(TreeNode* root){
        if(root == NULL)
            return;
        inorder(root->left);
        ivec.push_back(root->val);
        inorder(root->right);
    }
};
           

思路2：非遞歸實作。需要用一個輔助棧。

C++代碼實作：

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ivec;
        stack<TreeNode*> st;
        if(root == NULL)
            return ivec;
        TreeNode *p = root;
        while(p || !st.empty()){
            if(p){
                st.push(p);
                p = p ->left;
            }
            else{
                if(!st.empty()){
                    p = st.top();
                    st.pop();
                    ivec.push_back(p->val);
                    p = p->right;
                }
            }
        }
        return ivec;
    }
};