105 Construct Binary Tree from Preorder and Inorder Traversal

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

Solution

/**
 * 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* help(vector<int>& preorder, int ps, int pe, vector<int>& inorder, int is, int ie) {
        if ( ps > pe ) return NULL;
        int i;
        for ( i = is; i <= ie; i++ ) if ( inorder[i] == preorder[ps] ) break;
        TreeNode *ptree = new TreeNode(preorder[ps]);
        ptree->left = help(preorder, ps+1, ps+i-is, inorder, is, i-1);
        ptree->right = help(preorder, ps+i-is+1, pe, inorder, i+1, ie);
        return ptree;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        return help(preorder, 0, preorder.size()-1, inorder, 0, inorder.size()-1);
    }
};