lintcode: Construct Binary Tree from Preorder and Inorder Traversal

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

我以前的博文已有介绍​​构造二叉树​​

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */


class Solution {
    /**
     *@param preorder : A list of integers that preorder traversal of a tree
     *@param inorder : A list of integers that inorder traversal of a tree
     *@return : Root of a tree
     */
public:
    TreeNode *helper(vector<int> &preorder, int pL,int pR,vector<int> &inorder,int iL,int iR){
        TreeNode *node=new TreeNode(preorder[pL]);

        vector<int>::iterator it=find(inorder.begin(),inorder.end(),preorder[pL]);
        int mid=it-inorder.begin();

        if(mid==iL){
            node->left=NULL;
        }else{
            node->left=helper(preorder,pL+1,pL+(mid-iL),inorder,iL,mid-1);
        }

        if(mid==iR){
            node->right=NULL;
        }else{
            node->right=helper(preorder,pL+(mid-iL)+1,pR,inorder,mid+1,iR);
        }

        return node;
    }


    TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
        // write your code here
        int len=preorder.size();
        if(len==0){
            return NULL;
        }

        return helper(preorder,0,len-1,inorder,0,len-1);
    }
};