[LeetCode]105 Construct Binary Tree from Preorder and Inorder Traversal

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode buildTree(int[] preorder, int[] inorder)
{
int len = preorder.length;
Map<Integer, Integer> inmap = new HashMap<>();
for (int i = 0 ; i < len ; i ++)
{
inmap.put(inorder[i], i);
}

return help(preorder, 0, len - 1, inmap, 0, len - 1);
}

private TreeNode help(int[] preorder, int ps, int pe, Map<Integer, Integer> inmap, int is, int ie)
{
if (ps > pe || is > ie)
return null;

// Root is preorder[ps];
TreeNode root = new TreeNode(preorder[ps]);

int inpos = inmap.get(root.val);

int leftnodes = inpos - is; // How many nodes on the left of root.
root.left = help(preorder, ps + 1, ps + leftnodes, inmap, is, inpos - 1);
root.right = help(preorder, ps + leftnodes + 1, pe, inmap, inpos + 1, ie);
return root;
}
}