Wins the offer-32. Binary search tree with a doubly linked list (191)

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32. The binary search trees and doubly-linked list (191)

• Description Title: input a binary search tree, converts the binary search tree into a doubly linked list sorted. Requirements can not create any new node, point to only adjust the tree node pointer.

• Ideas: The nodes in preorder tree. For the current node, we need to change the current node leftpoints to its previous nodes, so that before a node rightpoints to the current node.

• Code:

  package _32.二叉搜索树与双向链表;
  
  import java.util.Stack;
  
  /**
   * 题目描述：输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的双向链表。
   * 要求不能创建任何新的结点，只能调整树中结点指针的指向。
   * @author Administrator
   *
   */
  public class ConvertBSTtoLink {
  	/**
  	 * 采用中序遍历结点：对于每一个结点，需要更改前一个结点的right(后继结点)，
  	 * 和当前结点的left(前驱结点)
  	 * @param pRootOfTree
  	 * @return
  	 */
      public static TreeNode Convert(TreeNode pRootOfTree) {
         if(pRootOfTree == null){
      	   return null;
         }
         
         Stack<TreeNode> stack = new Stack<>();
         //保存当前结点的前一个结点，以便给前一结点的 right赋值
         TreeNode pre = null;
         //中序遍历结点
         TreeNode cur = pRootOfTree;
         //保存链表的头结点
         TreeNode linkedHead = null;
         
         boolean firstFlag = true;
        
         while(!stack.isEmpty() || cur != null){
      	   while(cur != null){
      		   stack.push(cur);
      		   cur = cur.left;
      	   }
      	   if(cur == null){
      		   cur = stack.pop();
      		   if(firstFlag){
      			   linkedHead = cur;
      			   pre = cur;
      			   firstFlag = false;
      		   }
      		   else{
      			   pre.right = cur;
      			   cur.left = pre;
      			   pre = cur;
      		   }
      		   cur = cur.right;
      	   }
         }
         return linkedHead;
      }
      
      public static void main(String[] args) {
      	  //          10
          //          /   \
          //         6     14
          //       /  \   / \
          //      4    8 12  16
          TreeNode root = new TreeNode(10);
          root.left = new TreeNode(6);
          root.right = new TreeNode(14);
          root.left.left = new TreeNode(4);
          root.left.right = new TreeNode(8);
          root.right.left = new TreeNode(12);
          root.right.right = new TreeNode(16);
          TreeNode result = Convert(root);
          while(result != null){
          	 System.out.println(result);
          	 result = result.right;
          }
  	}
  }
  class TreeNode {
      int val = 0;
      TreeNode left = null;
      TreeNode right = null;
  
      public TreeNode(int val) {
          this.val = val;
      }
  
  	@Override
  	public String toString() {
  		return "TreeNode [val=" + val + "]";
  	}
  }