前序遍歷-根左右:
循環到左子樹空,else轉右子樹,當一個結點已經訪問右子樹時,該結點出棧
#include<iostream>
#include<string>
#include<stack>
#include<sstream>
#include<vector>
using namespace std;
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> ans;
stack<TreeNode*> treestack;
TreeNode* tem = root;
while(!treestack.empty()||tem){//棧空防止指針指空
if(tem){
ans.push_back(tem->val);//前序在這裏進行內容的讀取
treestack.push(tem);//在這裏入棧,減少判斷次數
tem = tem->left;
}//左子樹到底了
else{
tem = treestack.top();
treestack.pop();//左空就算結束,要彈出
tem = tem->right;
}
}
return ans;
}
};
int main(){
TreeNode* root = new TreeNode(6);
root->left = new TreeNode(2);
root->left->left = new TreeNode(1);
root->left->right = new TreeNode(4);
root->left->right->left = new TreeNode(3);
root->left->right->right = new TreeNode(5);
root->right = new TreeNode(7);
root->right->right = new TreeNode(8);
root->right->right->left = new TreeNode(9);
Solution s;
vector<int> v = s.preorderTraversal(root);
for(int i=0;i<v.size();i++){
cout<<v[i]<<endl;
}
return 0;
}
中序遍歷-左根右:
原理同前序,因爲總是訪問右子樹時出棧,意味着出棧結點左子樹空,即我們要訪問的結點
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> ans;
stack<TreeNode*> treestack;
TreeNode* tem = root;
while(!treestack.empty()||tem){//棧空防止指針指空
if(tem){
treestack.push(tem);//在這裏入棧,減少判斷次數
tem = tem->left;
}//左子樹到底了
else{
tem = treestack.top();
treestack.pop();//左空就算結束,要彈出
ans.push_back(tem->val);//中序在這裏進行內容的讀取
tem = tem->right;
}
}
return ans;
}
};
後序遍歷-左右根:
後序非遞歸遍歷一般有兩種思路
①變左右根爲根右左再取反
即先類似前序遍歷得到根右左的結果,然後再對結果取反,思路是簡單可行的,但是不方便對樹進行其他操作,只能看遍歷結果
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> ans,fin;
stack<TreeNode*> treestack;
TreeNode* tem = root;
while(!treestack.empty()||tem){//棧空防止指針指空
if(tem){
ans.push_back(tem->val);
treestack.push(tem);//在這裏入棧,減少判斷次數
tem = tem->right;
}//左子樹到底了
else{
tem = treestack.top();
treestack.pop();//左空就算結束,要彈出
tem = tem->left;
}
}
for(int i=ans.size()-1;i>=0;i--){
fin.push_back(ans[i]);
}
return fin;
}
};
②當前結點沒有孩子或孩子都被訪問時訪問此結點
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
TreeNode* tem;//注意這裏不需要初始化
stack<TreeNode*> treestack;
treestack.push(root);//根要先入棧,這一點與前面不同
vector<int> ans;
TreeNode* pre = NULL;
while(!treestack.empty()){//判斷條件只需要棧不空
tem = treestack.top();
if((tem->left==NULL&&tem->right==NULL)||(pre!=NULL&&(pre==tem->left||pre==tem->right))){
//子樹空或子樹都已經被訪問過
ans.push_back(tem->val);
treestack.pop();
pre = tem;
}
else{
if(tem->right!=NULL)treestack.push(tem->right);//先右子樹入棧
if(tem->left!=NULL)treestack.push(tem->left);//再左子樹入棧,實際上也是根右左,只不過棧是後進先出
}
}
return ans;
}
};