144. 二叉树的前序遍历
144. 二叉树的前序遍历
递归 解法
cpp
复制代码
/**
* 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> ans;
dfs(ans, root);
return ans;
}
void dfs(vector<int>& ans, TreeNode* root) {
if (root == nullptr)
return;
ans.push_back(root->val);
dfs(ans, root->left);
dfs(ans, root->right);
return;
}
};
非递归 解法
cpp
复制代码
/**
* 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) {}
* };
*/
//思路:
//先序遍历就是第一次到这个节点的时候,直接访问,再分别压入右节点,左节点
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
if(root==nullptr)return {};
vector<int>ans;
stack<TreeNode*>st;
st.push(root);
while(!st.empty() ){
TreeNode*temp=st.top();
st.pop();
ans.push_back(temp->val);
if(temp->right)st.push(temp->right);
if(temp->left)st.push(temp->left);
}
return ans;
}
};
94. 二叉树的中序遍历
94. 二叉树的中序遍历
递归 解法
cpp
复制代码
/**
* 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> ans;
dfs(ans, root);
return ans;
}
void dfs(vector<int>& ans, TreeNode* root) {
if (root == nullptr)
return;
dfs(ans, root->left);
ans.push_back(root->val);
dfs(ans, root->right);
return;
}
};
非递归 解法
cpp
复制代码
/**
* 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) {}
* };
*/
//思路 :
//一直进左子树,直到左子树为空
//访问栈顶
//再入栈的右节点
class Solution {
public:
vector<int> inorderTraversal(TreeNode* root) {
if(root==nullptr)return {};
vector<int>ans;
stack<TreeNode*>st;
TreeNode*cur=root;
while(cur||!st.empty() ){
while(cur){
st.push(cur);
cur=cur->left;
}
TreeNode*temp=st.top();
st.pop();
ans.push_back(temp->val);
cur=temp->right;
}
return ans;
}
};
145. 二叉树的后序遍历
145. 二叉树的后序遍历
递归 解法
cpp
复制代码
/**
* 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> ans;
dfs(ans, root);
return ans;
}
void dfs(vector<int>& ans, TreeNode* root) {
if (root == nullptr)
return;
dfs(ans, root->left);
dfs(ans, root->right);
ans.push_back(root->val);
return;
}
};
非递归 解法
cpp
复制代码
/**
* 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) {}
* };
*/
//思路 :
//先序是 中 左 右
//可以改为 中 右 左
//再倒过来 左 右 中
class Solution {
public:
vector<int> postorderTraversal(TreeNode* root) {
if(root==nullptr)return {};
stack<TreeNode*>st;
vector<int>ans;
st.push(root);
while(!st.empty() ){
TreeNode*temp=st.top();
ans.push_back(temp->val);
st.pop();
if(temp->left)st.push(temp->left);
if(temp->right)st.push(temp->right);
}
reverse(ans.begin(),ans.end() );
return ans;
}
};
102. 二叉树的层序遍历
102. 二叉树的层序遍历
队列 解法
cpp
复制代码
/**
* 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) {}
* };
*/
//思路 :
//使用队列记录要遍历的节点
class Solution {
public:
vector<vector<int>> levelOrder(TreeNode* root) {
if (root == nullptr)
return {};
queue<TreeNode*> que;
que.push(root);
vector<vector<int>> ans;
while (!que.empty()) {
vector<int> arr;
int N = que.size();
while (N--) {
TreeNode* temp = que.front();
que.pop();
arr.push_back(temp->val);
if (temp->left)
que.push(temp->left);
if (temp->right)
que.push(temp->right);
}
ans.push_back(arr);
}
return ans;
}
};