标签：-- post loop 直接 algorithm round back 写法 重点
PS:本文主要包括二叉树的前序，中序，后序，层序，垂序遍历，使用cpp语言。
前序遍历
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) {
stack<TreeNode*> sta;
vector<int> res; // 返回的数组
if (root == nullptr) return res; // 判断根节点,空树，直接返回
sta.push(root);
while (!sta.empty()) {
TreeNode* tmp = sta.top();
sta.pop();
if (tmp->right != nullptr) sta.push(tmp->right);
if (tmp->left != nullptr) sta.push(tmp->left);
res.push_back(tmp->val);
}
return res;
}
};
中序遍历
#include <iostream>
#include <vector>
#include <stack>
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> inorderTraversal(TreeNode* root) {
stack <TreeNode*> sta;
vector <int> res;
if (root == nullptr) return res; // 空树， 直接返回res
TreeNode* cur = root;
while (cur != nullptr || !sta.empty) {
// 先遍历左孩子
if (cur != nullptr ){
sta.push(cur);
cur = cur->left;
}
else {
// 再 是 根结点
TreeNode* temp = sta.top();
sta.pop(); // 访问完根节点后就可以出栈，左孩子和根节点都访问完
res.push_back(temp->val);
// 最后右孩子
cur = temp->right;
}
}
return res;
}
};
后序遍历
#include <iostream>
#include <vector>
#include <stack>
using namespace std;
struct TreeNode {
int val;
TreeNode* left;
TreeNode* right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
// 第一种写法
class Solution2 {
public:
vector<int> postorderTraversal(TreeNode* root) {
stack<TreeNode*> sta;
vector<int> res ;
if (root == nullptr) return res;
TreeNode* cur = root;
sta.push(cur);
while(!sta.empty()){
TreeNode* temp = sta.top();
res.push_back(temp->val);
sta.pop();
if (temp->left) sta.push(temp->left);
if (temp->right) sta.push(temp->right);
}
reverse(res.begin(),res.end());
return res;
}
};
// 第二种方法 (重点理解)
vector<int> PostOrderLoop(TreeNode* root)
{
stack<TreeNode*> sta;
vector<int> ans;
TreeNode* cur = root;
TreeNode* pre = nullptr;
while (cur || !sta.empty())
{
while (cur)
{
sta.push(cur);
cur = cur->left;
}
cur = sta.top();
//若右节点已经访问过或者没有右节点 则输出该节点值
if (cur->right == nullptr || pre == cur->right) {
sta.pop();
ans.push_back(cur->val);
pre = cur;
cur = nullptr;
}
else {
cur = cur->right;
pre = nullptr;
}
}
return ans;
}
层序遍历
#include <iostream>
#include <vector>
#include <queue>
using namespace std;
//Definition for a binary tree node.
struct TreeNode {
int val;
TreeNode* left;
TreeNode* right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
// BFS
class Solution {
public:
vector<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int>> res;
if (root == nullptr) return res;
queue<TreeNode*> que;
que.push(root);
while (!que.empty()) {
int size = que.size();
vector<int> level;
while (size--) {
TreeNode* temp = que.front();
que.pop();
level.push_back(temp->val);
if (temp->left != nullptr) que.push(temp->left);
if (temp->right != nullptr) que.push(temp->right);
}
res.push_back(level);
}
return res;
}
};
垂序遍历
#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;
// Definition for a binary tree node.
struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
class Solution {
public:
map<int, vector<pair<int, int>>> tree;
void dfs(TreeNode* root, int x, int y) {
if (root == nullptr) return;
tree[x].push_back(make_pair(y, root->val));
dfs(root->left, x - 1, y + 1); // 将节点进行给予坐标处理
dfs(root->right, x + 1, y + 1);
}
vector<vector<int>> verticalTraversal(TreeNode* root) {
vector<vector<int>> res;
if (root == nullptr) return res;
dfs(root, 0, 0);
for (auto& node : tree) { // 由于要修改node的值，需要加引用
sort(node.second.begin(), node.second.end());
vector<int> tmp;
for (auto t : node.second) {
tmp.push_back(t.second); // 获取val
}
res.push_back(tmp);
}
return res;
}
};
标签：-- post loop 直接 algorithm round back 写法 重点
原文地址：https://www.cnblogs.com/E-Dreamer-Blogs/p/14673476.html
