【Leetcode -101.对称二叉树 -144.二叉树的前序遍历】

		bool isSameTree(struct TreeNode* leftRoot, struct TreeNode* rightRoot)
		{
		    //都为空符合要求
		    if (leftRoot == NULL && rightRoot == NULL)
		        return true;
		
		    //一个为空一个不为空，不符合
		    if (leftRoot == NULL || rightRoot == NULL)
		        return false;
		
		    //值不相等不符合
		    if (leftRoot->val != rightRoot->val)
		        return false;
		
		    //左根的左子树和右根的右子树比较；左根的右子树和右根的左子树比较
		    return isSameTree(leftRoot->left, rightRoot->right) && isSameTree(leftRoot->right, rightRoot->left);
		}
		
		bool isSymmetric(struct TreeNode* root)
		{
		    if (root == NULL)
		        return true;
		
		    //比较根的左子树和右子树
		    return isSameTree(root->left, root->right);
		}

		//计算节点数
		int TreeSize(struct TreeNode* root)
		{
		    if (root == NULL)
		        return 0;
		
		    return TreeSize(root->left) + TreeSize(root->right) + 1;
		}
		
		
		void PrevOrder(struct TreeNode* root, int* ans, int* pi)
		{
		    //如果空返回
		    if (root == NULL)
		        return;
		
		    //先存放根的val，再左子树，后右子树
		    ans[(*pi)++] = root->val;
		    PrevOrder(root->left, ans, pi);
		    PrevOrder(root->right, ans, pi);
		}
		
		
		int* preorderTraversal(struct TreeNode* root, int* returnSize)
		{
		    //先计算节点的个数
		    *returnSize = TreeSize(root);
		    int* ans = (int*)malloc(*returnSize * sizeof(int));
		    int i = 0;
		
		    //前序遍历
		    PrevOrder(root, ans, &i);
		
		    return ans;
		}