[Algorithms] 二叉树的层序遍历
Categories Algorithms BinaryTree
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来源：代码随想录
二叉树的层序遍历
/**
* 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<vector<int>> levelOrder(TreeNode* root) {
queue<TreeNode *> que;
vector<int> v;
vector<vector<int>> vv;
if (root != nullptr) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
for (int i = 0; i < size; i++) {
TreeNode *node = que.front();
que.pop();
v.push_back(node->val);
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
vv.push_back(v);
v.clear();
}
return vv;
}
};二叉树的层次遍历 II
两次反转。
/**
* 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<vector<int>> levelOrderBottom(TreeNode* root) {
vector<vector<int>> vv;
queue<TreeNode *> que;
if (root != nullptr) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
vector<int> v;
for (int i = 0; i < size; i++) {
TreeNode *node = que.front();
que.pop();
v.push_back(node->val);
if (node->right) {
que.push(node->right);
}
if (node->left) {
que.push(node->left);
}
}
reverse(v.begin(), v.end());
vv.push_back(v);
}
reverse(vv.begin(), vv.end());
return vv;
}
};二叉树的右视图
单独处理size - 1。
/**
* 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> rightSideView(TreeNode* root) {
queue<TreeNode *> que;
vector<int> v;
if (root != nullptr) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
for (int i = 0; i < size; i++) {
TreeNode *node = que.front();
que.pop();
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
if (i == size - 1) {
v.push_back(node->val);
}
}
}
return v;
}
};二叉树的层平均值
/**
* 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<double> averageOfLevels(TreeNode* root) {
queue<TreeNode *> que;
vector<double> v;
if (root) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
double sum = 0;
for (int i = 0; i < size; i++) {
TreeNode *node = que.front();
que.pop();
sum += node->val;
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
v.push_back(sum / size);
}
return v;
}
};N 叉树的层序遍历
auto iter : node->children
/*
// Definition for a Node.
class Node {
public:
int val;
vector<Node*> children;
Node() {}
Node(int _val) {
val = _val;
}
Node(int _val, vector<Node*> _children) {
val = _val;
children = _children;
}
};
*/
class Solution {
public:
vector<vector<int>> levelOrder(Node* root) {
queue<Node *> que;
vector<vector<int>> vv;
if (root) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
vector<int> v;
for (int i = 0; i < size; i++) {
Node *node = que.front();
que.pop();
v.push_back(node->val);
for (auto iter : node->children) {
if (iter) {
que.push(iter);
}
}
}
vv.push_back(v);
}
return vv;
}
};在每个树行中找最大值
/**
* 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> largestValues(TreeNode* root) {
queue<TreeNode *> que;
vector<int> v;
if (root) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
int mmax = INT_MIN;
for (int i = 0; i < size; i++) {
TreeNode *node = que.front();
mmax = max(mmax, node->val);
que.pop();
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
v.push_back(mmax);
}
return v;
}
};填充每个节点的下一个右侧节点指针
LeetCode 116. 填充每个节点的下一个右侧节点指针
Node *nodeNext = nullptr;
if (!que.empty()) {
nodeNext = que.front();
}
node->next = nodeNext;如上的方式会出现问题，每一层最后一个节点的下一个节点被指向了下一层的第一个节点。所以采用标记前一个节点的方式。
/*
// Definition for a Node.
class Node {
public:
int val;
Node* left;
Node* right;
Node* next;
Node() : val(0), left(NULL), right(NULL), next(NULL) {}
Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}
Node(int _val, Node* _left, Node* _right, Node* _next)
: val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution {
public:
Node* connect(Node* root) {
queue<Node *> que;
if (root) {
que.push(root);
}
while (!que.empty()) {
int size = que.size();
Node *nodePrev = nullptr;
for (int i = 0; i < size; i++) {
Node *node = que.front();
if (nodePrev) {
nodePrev->next = node;
}
que.pop();
nodePrev = node;
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
}
return root;
}
};填充每个节点的下一个右侧节点指针 II
LeetCode 117. 填充每个节点的下一个右侧节点指针 II
和上一道一模一样。
/*
// Definition for a Node.
class Node {
public:
int val;
Node* left;
Node* right;
Node* next;
Node() : val(0), left(NULL), right(NULL), next(NULL) {}
Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}
Node(int _val, Node* _left, Node* _right, Node* _next)
: val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution {
public:
Node* connect(Node* root) {
queue<Node *> que;
if (root) {
que.push(root);
}
while (!que.empty()) {
Node *nodePrev = nullptr;
int size = que.size();
while (size--) {
Node *node = que.front();
if (nodePrev) {
nodePrev->next = node;
}
que.pop();
nodePrev = node;
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
}
return root;
}
};二叉树的最大深度
注意深度和高度的区别，最大深度指的是从根节点到最远叶子节点的最长路径上的节点数。
/**
* 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:
int maxDepth(TreeNode* root) {
queue<TreeNode *> que;
if (root) {
que.push(root);
}
int depth = 0;
while (!que.empty()) {
int size = que.size();
depth++;
while (size--) {
TreeNode *node = que.front();
que.pop();
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
}
return depth;
}
};二叉树的最小深度
/**
* 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:
int minDepth(TreeNode* root) {
queue<TreeNode *> que;
if (root) {
que.push(root);
}
int depth = 0;
while (!que.empty()) {
int size = que.size();
depth++;
while (size--) {
TreeNode *node = que.front();
que.pop();
if (!node->right && !node->left) {
return depth;
}
if (node->left) {
que.push(node->left);
}
if (node->right) {
que.push(node->right);
}
}
}
return depth;
}
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