面试：C++二叉树遍历（递归/非递归）

 #include <vector>
 #include <stack>
 #include <queue>
 using namespace std;

 struct TreeNode{
     int val;
     TreeNode* left;
     TreeNode* right;
     TreeNode(int x):val(x),left(nullptr),right(nullptr){};
 };

 void preOrder(TreeNode* root,vector<int>& res){
     if(root == nullptr) return;
     res.push_back(root->val);
     preOrder(root->left,res);
     preOrder(root->right,res);
 }

 void inOrder(TreeNode* root,vector<int>& res){
     if(root == nullptr) return;
     inOrder(root->left,res);
     res.push_back(root->val);
     inOrder(root->right,res);
 }

 void postOrder(TreeNode* root,vector<int>& res){
     if(root == nullptr) return;
     postOrder(root->left,res);
     postOrder(root->right,res);
     res.push_back(root->val);
 }

 vector<int> preOrder(TreeNode* root){
     vector<int> res;
     if(root == nullptr) return res;

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

 vector<int> inOrder(TreeNode* root){
     vector<int> res;
     if(root==nullptr) return res;

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

 vector<int> postOrder(TreeNode* root){
     vector<int> res;
     if(root == nullptr) return res;

     stack<TreeNode*> st;
     TreeNode* cur = root;
     while(cur){
         st.push(cur);
         cur = cur->left;
     }

     TreeNode* lastVisited = nullptr;
     while(!st.empty()){
         cur = st.top();
         st.pop();
         if(cur->right == nullptr || cur->right == lastVisited){
             res.push_back(cur->val);
             lastVisited = cur;
         }else{
             st.push(cur);
             cur = cur->right;
             while(cur){
                 st.push(cur);
                 cur = cur->left;
             }
         }
     }
     return res;
 }

 vector<int> levelOrder(TreeNode* root){
     vector<int> res;
     if(root == nullptr) return res;

     queue<TreeNode*> q;
     q.push(root);
     while(!q.empty()){
         size_t n = q.size();
         for(size_t i=;i<n;i++){
             TreeNode* cur = q.back();
             q.pop();
             res.push_back(cur->val);
             if(cur->left) q.push(cur->left);
             if(cur->right) q.push(cur->right);
         }
     }
     return res;
 }