Construct Binary Tree from Preorder and Inorder Traversal

 

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {    int findpos(vector<int> &vec,int s,int e,int val){        for(int i=s;i<=e;++i){            if(vec[i]==val)return i;        }        return -1;    }    TreeNode *Build(vector<int> &inorder,int si,int ei, vector<int> &preorder,int sp,int ep){        int val=preorder[sp];        TreeNode *root = new TreeNode(val);                int it= findpos(inorder,si,ei,val);        if(si>it-1)            root->left=NULL;        else            root->left  = Build(inorder,si,it-1,preorder,sp+1,sp+(it-si));                if(it+1>ei)            root->right=NULL;        else                root->right = Build(inorder,it+1,ei,preorder,ep-(ei-it)+1,ep);        return root;    }public:    TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(inorder.size()==0)return NULL;                return Build(inorder,0,inorder.size()-1,preorder,0,preorder.size()-1);            }};