刷题94. Binary Tree Inorder Traversal

一、题目说明

题目94. Binary Tree Inorder Traversal，给一个二叉树，返回中序遍历序列。题目难度是Medium！

二、我的解答

用递归遍历，学过数据结构的应该都可以实现。

class Solution{
    public:
        vector<int> inorderTraversal(TreeNode* root){
            if(root != NULL){
                if(root->left !=NULL)inorderTraversal(root->left);
                res.push_back(root->val);
                if(root->right !=NULL)inorderTraversal(root->right);
            }
            return res;
        }
    private:
        vector<int> res;
};

Runtime: 4 ms, faster than 61.00% of C++ online submissions for Binary Tree Inorder Traversal.
Memory Usage: 10.5 MB, less than 5.00% of C++ online submissions for Binary Tree Inorder Traversal.

三、优化措施

用非递归算法，需要一个栈，代码如下：

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

性能：

Runtime: 4 ms, faster than 60.93% of C++ online submissions for Binary Tree Inorder Traversal.
Memory Usage: 9.2 MB, less than 89.00% of C++ online submissions for Binary Tree Inorder Traversal.