105. Construct Binary Tree from Preorder and Inorder Traversal

题目：

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

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

解题思路：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        return buildTree_help(preorder,0,preorder.size()-1, inorder,0,inorder.size()-1);
    }
    TreeNode* buildTree_help(vector<int>& preorder,int start_pre,int end_pre, vector<int>& inorder,int start_in,int end_in){
        if(start_pre>end_pre)return NULL;
        TreeNode *root = new TreeNode(preorder[start_pre]);
        int i = find(inorder.begin()+start_in,inorder.begin()+end_in,root->val) - inorder.begin()-start_in;
        root->left = buildTree_help(preorder,start_pre+1,start_pre+i, inorder,start_in,start_in+i-1);
        root->right = buildTree_help(preorder,start_pre+i+1,end_pre, inorder,start_in+i+1,end_in);
        return root;
    }
};