[C++] LeetCode 二叉树的前序、中序、后序遍历

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题目

二叉树的前序、中序、后序遍历，采用递归和非递归写法。
具体题目见：
二叉树前序遍历https://leetcode-cn.com/problems/binary-tree-preorder-traversal/description/
二叉树中序遍历(https://leetcode-cn.com/problems/binary-tree-inorder-traversal/description/)
二叉树后序遍历(https://leetcode-cn.com/problems/binary-tree-postorder-traversal/description/)

前序遍历

递归写法

/**
 * 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:
    void preorder(TreeNode *root,vector<int> &res){
        if(root==NULL) return;
        res.push_back(root->val);
        preorder(root->left,res);
        preorder(root->right,res);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        preorder(root,res);
        return res;
    }
};

采用栈

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

采用栈 方法二

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

常数空间

/**
 * 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> preorderTraversal(TreeNode* root) {
        TreeNode *tmp=NULL;
        vector<int> res;
        while(root){
            tmp=root->left;
            while(tmp&&tmp->right!=root&&tmp->right!=NULL){
                tmp=tmp->right;
            }
            if(tmp==NULL){
                res.push_back(root->val);
                root=root->right;
            }
            else if(tmp->right==NULL){
                res.push_back(root->val);
                tmp->right=root;
                root=root->left;
            }
            else{
                tmp->right=NULL;
                root=root->right;
            }
        }
        return res;
    }
};

中序遍历

递归写法

/**
 * 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:
    void preorder(TreeNode *root,vector<int> &res){
        if(root==NULL) return;
        preorder(root->left,res);
        res.push_back(root->val);
        preorder(root->right,res);
    }
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        preorder(root,res);
        return res;
    }
};

采用栈

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

采用栈 方法二

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

常数空间

/**
 * 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) {
        TreeNode *tmp=NULL;
        vector<int> res;
        while(root){
            tmp=root->left;
            while(tmp&&tmp->right!=root&&tmp->right!=NULL){
                tmp=tmp->right;
            }
            if(tmp==NULL){
                res.push_back(root->val);
                root=root->right;
            }
            else if(tmp->right==NULL){
                tmp->right=root;
                root=root->left;
            }
            else{
                res.push_back(root->val);
                tmp->right=NULL;
                root=root->right;
            }
        }
        return res;
    }
};

后序遍历

递归写法

/**
 * 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:
    void preorder(TreeNode *root,vector<int> &res){
        if(root==NULL) return;
        preorder(root->left,res);
        preorder(root->right,res);
        res.push_back(root->val);
    }
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        preorder(root,res);
        return res;
    }
};

采用栈

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

常数空间

/**
 * 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> postorderTraversal(TreeNode* root) {
        vector<int> res;
        TreeNode *tmp=NULL;
        while(root){
            tmp=root->right;
            while(tmp&&tmp->left!=NULL&&tmp->left!=root){
                tmp=tmp->left;
            }
            if(tmp==NULL){
                res.push_back(root->val);
                root=root->left;
            }
            else if(tmp->left==NULL){
                res.push_back(root->val);
                tmp->left=root;
                root=root->right;
            }
            else{
                tmp->left=NULL;
                root=root->left;
            }
        }
        reverse(res.begin(),res.end());
        return res;
    }
};