BuildTreeByPreAndInorder

Author: jackzhenguo

Tree/Tree.TreeLib/BuildTreeByPreAndInorder.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Tree.TreeLib
+{
+    // // Given preorder and inorder traversal of a tree, construct the binary tree.
+
+// // Note: 
+// // You may assume that duplicates do not exist in the tree.
+   public class Solution 
+{
+    private  int[] _preorder; //preorder array
+    private  int[] _inorder; //inorder array
+    public TreeNode BuildTree(int[] preorder, int[] inorder)
+     {
+        _preorder = preorder;
+        _inorder = inorder;
+        //if(_preorder.Length==0 || inorder.Length==0) return null;
+        //if(prorder.Length!=inorder.Length) return null;
+        return bulidTree(0,0,_inorder.Length-1);
+    }
+    //preStart: index of root of tree
+    //inStart: of inorder tree, inStart ~ root index - 1 -> left tree
+    //EndStart:of inorder tree, root index + 1 ~ inEnd -> right tree
+    private TreeNode  bulidTree(int preStart, int inStart, int inEnd) 
+    {
+        if (preStart > _preorder.Length - 1 || inStart > inEnd)
+         {
+            return null;
+        }
+        TreeNode root = new TreeNode(_preorder[preStart]); 
+        // find the index of current root in inorder. 
+        int inIndex = curRootIndex(inStart, inEnd, root); 
+        root.left = bulidTree(preStart + 1, inStart, inIndex - 1);
+        //right subtree begins position in preorder
+        preStart += inIndex - inStart + 1; 
+        root.right = bulidTree(preStart, inIndex + 1, inEnd);
+        return root;
+    }
+
+      private int curRootIndex(int inStart, int inEnd, TreeNode root)
+      {
+           for (int i = inStart; i <= inEnd; i++) 
+           {
+               if (_inorder[i] == root.val) 
+                {
+                   return i;
+                }
+            }
+          return -1;
+        }
+
+  }
+
+}