代码随想录：二叉树的四种遍历

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:vector<int> preorderTraversal(TreeNode* root) {vector<int> anwser;traval(root,anwser);return anwser;}void traval(TreeNode* root,vector<int>& anwser){if(!root)return ;anwser.push_back(root->val);traval(root->left,anwser);traval(root->right,anwser);return ;}
};

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:vector<int> postorderTraversal(TreeNode* root) {vector<int> anwser;traval(root,anwser);return anwser;}void traval(TreeNode *root,vector<int>& anwser){if(root==NULL)return ;traval(root->left,anwser);traval(root->right,anwser);anwser.push_back(root->val);return;}
};

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:vector<int> inorderTraversal(TreeNode* root) {vector<int>anwser;traval(root,anwser);return anwser;}void traval(TreeNode *root,vector<int> & anwser){if(root==NULL)return;traval(root->left,anwser);anwser.push_back(root->val);traval(root->right,anwser);return;}
};

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:vector<vector<int>> levelOrder(TreeNode* root) {vector<vector<int>> anwser;queue<TreeNode*> q;if(root)q.push(root);while(!q.empty()){int n=q.size();vector<int>layer;TreeNode* cur=q.front();for(int i=0;i<n;i++){TreeNode* cur=q.front();q.pop();if(cur->left)q.push(cur->left);if(cur->right)q.push(cur->right);layer.push_back(cur->val);}anwser.push_back(layer);}return anwser;}
};