1.单值二叉树

1.1法一：无返回值

struct TreeNode 
{int val;struct TreeNode *left;struct TreeNode *right;};bool flag = true;
void PreOrderCompare(struct TreeNode* root, int val)
{//递归过程中 遇NULL 或flag已变假 返回上一层if (root == NULL || flag == false)return;//遇非单值 更新flag 返回上一层if (root->val != val){flag = false;return;}//结点数据相等 继续遍历PreOrderCompare(root->left, val);PreOrderCompare(root->right, val);
}bool isUnivalTree(struct TreeNode* root)
{if (root == NULL)return true;else{//OJ题目会用程序测试多组样例//若上个样例flag变为false 这里就会出错//同时也提醒我们要慎用全局变量flag = true;PreOrderCompare(root, root->val);return flag;}
}

1.2法二：有返回值

根与左子树、右子树比较 不断递归

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

2.相同的树

struct TreeNode 
{int val;struct TreeNode *left;struct TreeNode *right;};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);
}

3.对称二叉树

struct TreeNode 
{int val;struct TreeNode *left;struct TreeNode *right;};bool isSymmetricSubTree(struct TreeNode* root1, struct TreeNode* root2)
{if (root1 == NULL && root2 == NULL)return true;if (root1 == NULL || root2 == NULL)return false;if (root1->val != root2->val)return false;return isSymmetricSubTree(root1->left, root2->right)&& isSymmetricSubTree(root1->right, root2->left);
}bool isSymmetric(struct TreeNode* root)
{if (root == NULL)return true;return isSymmetricsubTree(root->left, root->right);
}

4.二叉树的前序遍历

struct TreeNode 
{int val;struct TreeNode *left;struct TreeNode *right;};//计算树的结点个数
int TreeSize(struct TreeNode* root)
{return root == NULL ? 0 : TreeSize(root->left) + TreeSize(root->right) + 1;
}
//前序遍历
void preorder(struct TreeNode* root, int* a, int* i)
{if (root == NULL)return;a[(*i)++] = root->val;preorder(root->left, a, i);preorder(root->right, a, i);
}
int* preorderTraversal(struct TreeNode* root, int* returnSize)
{//调用TreeSize函数决定开多大的空间*returnSize = TreeSize(root);//开空间int* a = (int*)malloc(*returnSize * sizeof(int));//若在子函数用局部变量i -- 在下一层递归改变i后 -- 返回到上一层用的仍是旧i -- 出现错误int i = 0;preorder(root, a, &i);return a;
}

5.二叉树的中序遍历

struct TreeNode
{int val;struct TreeNode* left;struct TreeNode* right;
};int TreeSize(struct TreeNode* root)
{return root == NULL ? 0 : TreeSize(root->left) + TreeSize(root->right) + 1;
}
void inorder(struct TreeNode* root, int* a, int* i)
{if (root == NULL)return;inorder(root->left, a, i);a[(*i)++] = root->val;inorder(root->right, a, i);
}
int* inorderTraversal(struct TreeNode* root, int* returnSize)
{*returnSize = TreeSize(root);int* a = (int*)malloc(*returnSize * sizeof(int));int i = 0;inorder(root, a, &i);return a;
}

6.二叉树的后序遍历

struct TreeNode
{int val;struct TreeNode* left;struct TreeNode* right;
};int TreeSize(struct TreeNode* root)
{return root == NULL ? 0 : TreeSize(root->left) + TreeSize(root->right) + 1;
}
void posorder(struct TreeNode* root, int* a, int* i)
{if (root == NULL)return;posorder(root->left, a, i);posorder(root->right, a, i);a[(*i)++] = root->val;
}
int* posorderTraversal(struct TreeNode* root, int* returnSize)
{*returnSize = TreeSize(root);int* a = (int*)malloc(*returnSize * sizeof(int));int i = 0;posorder(root, a, &i);return a;
}

7.另一棵树的子树

struct TreeNode 
{int val;struct TreeNode *left;struct TreeNode *right;};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);
}

8.二叉树遍历

typedef char BTDataType;
typedef struct BinaryTreeNode
{struct BinaryTreeNode* left;struct BinaryTreeNode* right;BTDataType data;
}BTNode;//创建新结点
BTNode* CreatNode(BTDataType x)
{BTNode* node = (BTNode*)malloc(sizeof(BTNode));assert(node);node->data = x;node->left = NULL;node->right = NULL;return node;
}//建树
BTNode* CreateTree(char* str, int* i)
{if (str[*i] == '#'){(*i)++;return NULL;}//创建结点BTNode* root = CreatNode(str[(*i)++]);//连接子树root->left = CreateTree(str, i);root->right = CreateTree(str, i);return root;
}int main() 
{char str[100] = { 0 };scanf("%s", str);int i = 0;BTNode* root = CreateTree(str, &i);return 0;
}