[LeetCode]Construct Binary Tree from Preorder and Inorder Traversal

Given preorder and inorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

和前一题类似

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *help(vector<int> &preorder, int a, int b,vector<int> &inorder,int c , int d){
    	
		int len = b-a+1;
		if(len==0)
			return NULL;
		if(len==1)
		{
			TreeNode *p = new TreeNode(preorder[a]);
			return p;
		}
		//左根右
		//根左右
		int root = preorder[a];
		TreeNode *p = new TreeNode( root);
        int site;
         for(int i = c ; i <= d ; i++)
	         if (inorder[i]==root)
             {
                 site = i;
                 break;
             }
		int left_len = site - c;
        int right_len = len - left_len -1;
		if(left_len > 0)
			p->left = help(preorder,a+1,a+left_len,inorder,c, c + left_len-1);
		if(right_len > 0)
			p->right = help(preorder,a+left_len+1, b,inorder,site+1, d);
		return p;
		
	}
    TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
		size_t len = inorder.size();
		if(len == 0)
			return NULL;
       TreeNode *p = help(preorder,0,len-1,inorder,0,len-1);
	   return p;
    }
};