construct-binary-tree-from-preorder-

/**
 * Definition for binary tree
 * 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) {
        if(preorder.size()!=inorder.size())
            return NULL;
         
        return helper(preorder,inorder,0,preorder.size()-1,0,inorder.size()-1);
    }
    
    TreeNode *helper(vector<int> &preorder,vector<int> &inorder,int preStart,int preEnd,
                     int inStart,int inEnd)
    {
        if(preStart > preEnd || inStart > inEnd) {
            return NULL;
        }
        TreeNode *ROOT = new TreeNode(preorder[preStart]);
        int index = 0;
        for(int i = inStart; i <= inEnd; i++){
            if(inorder[i] == preorder[preStart]) {
                index = i;
                break;
            }
        }
        ROOT->left = helper(preorder, inorder, preStart + 1, preStart + index - inStart, inStart, index - 1);
        ROOT->right = helper(preorder, inorder, preStart + index - inStart + 1, preEnd, index + 1, inEnd);
        return ROOT;
    }
    
};