刷题105. Construct Binary Tree from Preorder and Inorder Traversal

一、题目说明

题目105. Construct Binary Tree from Preorder and Inorder Traversal，给二叉树的前序和中序遍历序列，构造一棵二叉树。题目难度是Medium！

二、我的解答

这个题目数据结构上面也有讲，这里用递归遍历算法。前序遍历第1个为树的根，然后用根将中序遍历分成左右子树，再递归就可以了。

代码如下：

class Solution{
	public:
		TreeNode* createTree(vector<int> & preorder,vector<int>& inorder,int leftStart,int leftEnd){
		    //preorder中当前元素为树根
			TreeNode* r = new TreeNode(preorder[curRoot]);
			
			int k=leftStart;
			while(inorder[k] !=preorder[curRoot]) k++;
			
			curRoot++;
			if(k>leftStart){
				r->left = createTree(preorder,inorder,leftStart,k-1);
			}
			if(k<leftEnd){
				r->right = createTree(preorder,inorder,k+1,leftEnd);
			}
			return r;
		}
		TreeNode* buildTree(vector<int>& preorder,vector<int>& inorder){
			int len = preorder.size();
			if(preorder.size()<1) return NULL;
			TreeNode* root = new TreeNode(preorder[0]);
			if(len==1) return root;
			
			int k=0;
			while(inorder[k] !=preorder[0]) k++;
			curRoot = 1;
			if(k>0){
				root->left = createTree(preorder,inorder,0,k-1);
			}
			if(k<len-1){
				root->right = createTree(preorder,inorder,k+1,len-1);
			}
			
			return root;
		}
	private:
		int curRoot;
};

性能如下：

Runtime: 24 ms, faster than 44.15% of C++ online submissions for Construct Binary Tree from Preorder and Inorder Traversal.
Memory Usage: 22.2 MB, less than 9.52% of C++ online submissions for Construct Binary Tree from Preorder and Inorder Traversal.

三、优化措施

代码简化如下：

class Solution{
	public:
		TreeNode* createTree(vector<int> & preorder,vector<int>& inorder,int leftStart,int leftEnd){
			TreeNode* r = new TreeNode(preorder[curRoot]);
			
			int k=leftStart;
			while(inorder[k] !=preorder[curRoot]) k++;
			
			curRoot++;
			if(k>leftStart){
				r->left = createTree(preorder,inorder,leftStart,k-1);
			}
			if(k<leftEnd){
				r->right = createTree(preorder,inorder,k+1,leftEnd);
			}
			return r;
		}
		TreeNode* buildTree(vector<int>& preorder,vector<int>& inorder){
			int len = preorder.size();
			if(len<1) return NULL;
			curRoot = 0;
			return createTree(preorder,inorder,0,len-1);
		}
	private:
		int curRoot;
};

Runtime: 20 ms, faster than 64.01% of C++ online submissions for Construct Binary Tree from Preorder and Inorder Traversal.
Memory Usage: 22.4 MB, less than 9.52% of C++ online submissions for Construct Binary Tree from Preorder and Inorder Traversal.