public TreeNode buildTree(int[] preorder, int[] inorder) { return helper(preorder,inorder,0,preorder.length-1,0,inorder.length-1); } public TreeNode helper(int[] preorder, int[] inorder, int pstart , int pend , int istart , int iend) { if(pstart > pend) { return null; } TreeNode root = new TreeNode(preorder[pstart]); int index = 0; while(inorder[index] != root.val) { index++; } root.left = helper(preorder,inorder,pstart+1,pstart+index - istart,istart,index-1); root.right = helper(preorder,inorder,pstart+index -istart +1,pend,index+1,iend); return root; }