C++实现——二叉树的四种遍历（非递归写法）

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

//结构体定义如下
typedef struct TreeNode{

    //权值
    int val;
    //左右孩子
    TreeNode* left, *right;
    //构造函数
    TreeNode() :left(NULL), right(NULL), val(0){}
    TreeNode(int val) :left(NULL), right(NULL), val(val){}

}TreeNode;

//二叉树的四种遍历（先序、中序、后序、层次）
//层次遍历
vector<int> layerTravel(TreeNode* root){

    vector<int> res;
    if (root == NULL)return res;
    queue<TreeNode*> q;
    q.push(root);
    while (!q.empty()){
        //计算当前队列中含有多少元素
        int size = q.size();
        //从头部依次取出size个元素
        for (int i = 0; i < size; i++){
            root = q.front();
            q.pop();
            res.push_back(root->val);
            //将下一层的元素塞入队列
            if (root->left)q.push(root->left);
            if (root->right)q.push(root->right);
        }

    }
    return res;
}

//先序和中序存在极大的相似性
//先序遍历
vector<int> preOrderTravel(TreeNode* root){

    vector<int> res;
    if (root == NULL)return res;
    stack<TreeNode*> s;
    while (root || !s.empty()){

        while (root){
            res.push_back(root->val);
            s.push(root);
            root = root->left;
        }
        root = s.top();
        s.pop();
        root = root->right;

    }

    return res;
}

//中序遍历
vector<int> inorderTravel(TreeNode* root){

    vector<int> res;
    if (root == NULL)return res;
    stack<TreeNode*>s;
    while (root || !s.empty()){

        while (root){
            s.push(root);
            root = root->left;
        }
        root = s.top();
        s.pop();
        res.push_back(root->val);
        root = root->right;
    }
    return res;
}

//后续遍历
vector<int> postOrderTravel(TreeNode* root){

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

    }
    return res;
}


int main(){

    return 0;
}

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