二叉树经典14题——初学二叉树必会的简单题

此篇皆为leetcode、牛客中的简单题型和二叉树基础操作，无需做过多讲解，仅付最优解。有需要的小伙伴直接私信我~

目录

1.二叉树的节点个数

2.二叉树叶子节点个数

3.二叉树第K层节点个数

4.查找值为X的节点

5.leetcode——二叉树的最大深度

6.leetcode——单值二叉树

7.leetcode——相同的树

8.二叉树的前序遍历

9.二叉树的中序遍历 

10.二叉树的后序遍历 

11.二叉树的层序遍历

12.leetcode——另一棵树的子树

13.二叉树的构建及遍历 

14.leetcode——对称二叉树

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1.二叉树的节点个数

int BinaryTreeSize(BTNode* root)
{return root == NULL ? 0 : BinaryTreeSize(root->left) + BinaryTreeSize(root->right) + 1;
}

2.二叉树叶子节点个数

int BinaryTreeLeafSize(BTNode* root)
{if (root == NULL)return 0;if (root->left == NULL && root->right == NULL)return 1;return BinaryTreeLeafSize(root->left) + BinaryTreeLeafSize(root->right);
}

3.二叉树第K层节点个数

int BinaryTreeLevelKSize(BTNode* root, int k)
{if (root == NULL)return 0;if (k == 1)return 1;return BinaryTreeLevelKSize(root->left, k - 1) + BinaryTreeLevelKSize(root->right, k - 1);
}

4.查找值为X的节点

BTNode* BinaryTreeFind(BTNode* root, BTDataType data)
{if (root == NULL)return NULL;if (root->data == data)return root;BTNode* ret1 = BinaryTreeFind(root->left, data);if (ret1)return ret1;BTNode* ret2 = BinaryTreeFind(root->right, data);if (ret2)return ret2;return NULL;
}

5.leetcode——二叉树的最大深度

oj链接：二叉树的最大深度

int maxDepth(struct TreeNode* root) {if (root == NULL)return 0;int leftDepth = maxDepth(root->left);int rightDepth = maxDepth(root->right);return  (leftDepth > rightDepth ? leftDepth : rightDepth) + 1;
}

6.leetcode——单值二叉树

oj链接：单值二叉树

bool isUnivalTree(struct TreeNode* root)
{if (root == NULL){return true;}if (root->left && root->val != root->left->val)return false;if (root->right && root->val != root->right->val)return false;return isUnivalTree(root->left) && isUnivalTree(root->right);
}

7.leetcode——相同的树

oj链接：相同的树

bool isSameTree(struct TreeNode* p, struct TreeNode* q) {if (p == NULL && q == NULL)return true;if (p == NULL || q == NULL)return false;if (p->val != q->val){return false;}return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
}

8.二叉树的前序遍历

详细讲解：二叉树的前、中、后序遍历

void PrevOrder(BTNode* root)
{if (root == NULL){printf("NULL ");return;}printf("%d ", root->data);//前序在前PrevOrder(root->left);PrevOrder(root->right);
}

9.二叉树的中序遍历 

void InOrder(BTNode* root)
{if (root == NULL){printf("NULL ");return;}InOrder(root->left);printf("%d ", root->data);//中序在中InOrder(root->right);
}

10.二叉树的后序遍历 

void PostOrder(BTNode* root)
{if (root == NULL){printf("NULL ");return;}PostOrder(root->left);PostOrder(root->right);printf("%d ", root->data);//后序在后
}

11.二叉树的层序遍历

详细讲解：看完这篇我不信你不会二叉树的层序遍历

//需自己实现队列的数据结构
void BinaryTreeLevelOrder(BTNode* root)
{Queue q;QueueInit(&q);if (root)QueuePush(&q, root);elsereturn;while (!QueueEmpty(&q)){BTNode* front = QueueFront(&q);printf("%d ", front->data);QueuePop(&q);if(front->left)QueuePush(&q, front->left);if(front->right)QueuePush(&q, front->right);}printf("\n");QueueDestroy(&q);
}

12.leetcode——另一棵树的子树

oj链接：另一棵树的子树

//判断两树是否相同（复用“相同的树”解题代码）
bool isSametree(struct TreeNode* p,struct TreeNode* q)
{if(p==NULL && q==NULL)return true;if(p==NULL || q==NULL)return false;if(p->val != q->val)return false;return isSametree(p->left,q->left) && isSametree(p->right,q->right);
}bool isSubtree(struct TreeNode* root, struct TreeNode* subRoot){if(root==NULL)return false;if(isSametree(root,subRoot))return true;return isSubtree(root->left,subRoot) || isSubtree(root->right,subRoot);
}

13.二叉树的构建及遍历 

题目链接：二叉树的遍历

注意：本题虽然是二叉树的遍历，但考查的却是如何通过数组的内容构建一棵二叉树。

#include <stdio.h>
#include <stdlib.h>struct TreeNode {int val;struct TreeNode *left;struct TreeNode *right;};struct TreeNode* rebuildTree(char* str,int* pi)
{if(str[*pi] == '#'){(*pi)++;return NULL;}struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));root->val = str[(*pi)++];root->left = rebuildTree(str,pi);root->right = rebuildTree(str,pi);return root;
}void TreeDestory(struct TreeNode* root)
{if(root==NULL)return;TreeDestory(root->left);TreeDestory(root->right);free(root);
}void InOrder(struct TreeNode* root)
{if (root == NULL){return;}InOrder(root->left);printf("%c ", root->val);InOrder(root->right);
}
int main() {char str[100];scanf("%s",str);int i=0;struct TreeNode* root = rebuildTree(str,&i);InOrder(root);TreeDestory(root);return 0;
}

14.leetcode——对称二叉树

oj链接：对称二叉树

bool _isSymmetric(struct TreeNode* p, struct TreeNode* q)
{if (p == NULL && q == NULL)return true;if (p == NULL || q == NULL)return false;if (p->val != q->val)return false;return _isSymmetric(p->left, q->right) &&_isSymmetric(p->right, q->left);
}
bool isSymmetric(struct TreeNode* root) {return root == NULL ? 0 : _isSymmetric(root->left, root->right);
}