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.