二叉樹遍歷(先序、中序、後序) 遞歸和非遞歸 c++

參考https://www.jianshu.com/p/456af5480cee

一、代碼

#include<iostream>
#include<stack>
#include<vector>
#include<algorithm>
using namespace std;

//可參考 https://www.jianshu.com/p/456af5480cee
//二叉樹遍歷(先序、中序、後序) 遞歸和非遞歸

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

//先序遍歷 遞歸
void preVisit1(TreeNode* root){
    if(!root){
        return;
    }
    cout << root->val << " ";
    preVisit1(root->left);
    preVisit1(root->right);
}
//先序遍歷 非遞歸
void preVisit2(TreeNode* root){
    stack<TreeNode*> s;
    vector<int> vec;
    TreeNode* node = root;
    while(node || !s.empty()){
        while(node){
            s.push(node);
            vec.push_back(node->val);
            node = node->left;
        }//退出條件爲node空（左子節點遍歷到頭了），然後彈棧往右邊找一個節點
        if(!s.empty()){
            node = s.top();
            s.pop();
            node = node->right;
        }
    }
    for(int i=0;i<vec.size();i++){
        cout << vec[i] << " ";
    }
}

//中序遍歷  遞歸
void midVisit1(TreeNode* root){
    if(!root){
        return;
    }
    midVisit1(root->left);
    cout << root->val << " ";
    midVisit1(root->right);
}

//中序遍歷  非遞歸
void midVisit2(TreeNode* root){
    //和先序遍歷非遞歸非常相似
    TreeNode* node = root;
    stack<TreeNode*> s;
    vector<int> vec;
    while(node || !s.empty()){
        while(node){
            s.push(node);
            node = node->left;
        }
        if(!s.empty()){
            node = s.top();
            s.pop();
            vec.push_back(node->val);
            node = node->right;
        }
    }
    for(int i=0;i<vec.size();i++){
        cout << vec[i] << " ";
    }
}

//後序遍歷  遞歸
void postVisit1(TreeNode* root){
    if(!root){
        return;
    }
    postVisit1(root->left);
    postVisit1(root->right);
    cout << root->val << " ";
}

//後序遍歷  非遞歸1
//後序：左右根 ->逆序後變爲：根右左 ->和先序遍歷(根左右)非常相似
void postVisit2(TreeNode* root){
    TreeNode* node = root;
    stack<TreeNode*> s;
    vector<int> vec;
    while(node || !s.empty()){
        while(node){
            s.push(node);
            vec.push_back(node->val);
            node = node->right;
        }
        if(!s.empty()){
            node = s.top();
            s.pop();
            node = node->left;
        }
    }
    //將vec逆序輸出
    reverse(vec.begin(), vec.end());
    for(int i=0;i<vec.size();i++){
        cout << vec[i] << " ";
    }
}

//後序遍歷  非遞歸2
void postVisit3(TreeNode* root){
    TreeNode* node = root;
    TreeNode* last_visit = NULL;
    stack<TreeNode*> s;
    vector<int> vec;
    while(node || !s.empty()){
        while(node){
            s.push(node);
            node = node->left;
        }
        node = s.top();
        if(!node->right || node->right==last_visit){//如果右節點爲空，或則右子樹已經訪問完
            vec.push_back(node->val);
            s.pop();  //訪問節點後才pop
            last_visit = node;
            node = NULL;
        }else{
            node = node->right;
        }
    }
    for(int i=0;i<vec.size();i++){
        cout << vec[i] << " ";
    }
}

int main(){

    //構建一個二叉樹
    TreeNode* a = new TreeNode(1);
    TreeNode* b = new TreeNode(2);
    TreeNode* c = new TreeNode(3);
    TreeNode* d = new TreeNode(4);
    TreeNode* e = new TreeNode(5);
    TreeNode* f = new TreeNode(6);
    TreeNode* g = new TreeNode(7);
    TreeNode* h = new TreeNode(8);
    a->left = b;
    a->right = c;
    b->left = d;
    d->right = f;
    f->left = g;
    f->right = h;
    c->right = e;

    cout << "preVisit1: ";
    preVisit1(a);
    cout << "\npreVisit2: ";
    preVisit2(a);

    cout << "\n\nmidVisit1: ";
    midVisit1(a);
    cout << "\nmidVisit2: ";
    midVisit2(a);

    cout << "\n\npostVisit1: ";
    postVisit1(a);
    cout << "\npostVisit2: ";
    postVisit2(a);
    cout << "\npostVisit3: ";
    postVisit3(a);

}

二、圖解 -（非遞歸）棧中元素變化