Construct Binary Tree from Preorder and Inorder Traversal

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.

使用递归的方法，注意到先序的特点

/**
 * 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 helper(preorder,0,preorder.size(),inorder,0,inorder.size());
    }
    TreeNode* helper(vector<int>& preorder,int i,int j,vector<int>& inorder,int ii,int jj){
        if(i>=j||ii>=jj) return NULL;
        
        int mid = preorder[i];
        auto f = find(inorder.begin()+ii,inorder.begin()+jj,mid);
        int len =f-inorder.begin()-ii;
        TreeNode* root= new TreeNode(mid);
        root->left = helper(preorder,i+1,i+1+len,inorder,ii,ii+len);
        root->right = helper(preorder,i+1+len,j,inorder,ii+len+1,jj);
        return root;
    }
};