面试题27：二叉排序树转双向链表

最新推荐文章于 2019-03-28 20:43:13 发布

piziZhang2013

最新推荐文章于 2019-03-28 20:43:13 发布

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分类专栏：剑指offer 文章标签：二叉树面试题链表搜索

本文链接：https://blog.youkuaiyun.com/u013378469/article/details/18185141

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本文介绍如何将二叉排序树转换为双向链表，并提供了三种基于中序遍历的方法实现这一转换过程。每种方法都详细展示了递归转换的过程，包括返回双向链表的头结点、尾结点或是根据条件返回头结点或尾结点。

说明：

1.二叉排序树又名二叉搜索树；

2.二叉树转成的双向链表是排好序的；

3.总共有三种方法实现，但都是基于中序遍历。

(一)返回双向链表的头结点

// return the head of doubleLink
BinaryTreeNode convertToDoubleLink1(BinaryTreeNode currentRoot){
		if(currentRoot == null){
			return null;
		}
		// currentRoot not equal to null
		BinaryTreeNode leftHead = null;	
		if(currentRoot.left != null){
			leftHead = this.convertToDoubleLink1(currentRoot.left);
		}	
		if(leftHead != null){
			BinaryTreeNode leftTail = leftHead; 
			while(leftTail.right != null){
				leftTail = leftTail.right;
			}
			leftTail.right = currentRoot;
			currentRoot.left = leftTail;
		}
		
		BinaryTreeNode rightHead = null;
		if(currentRoot.right != null){
			rightHead = this.convertToDoubleLink1(currentRoot.right);
		}
		if(rightHead != null){
			rightHead.left = currentRoot;
			currentRoot.right = rightHead;
		}
		return leftHead != null? leftHead : currentRoot;
}

(二)返回双向链表的尾结点

//return the tail of doubleLink
BinaryTreeNode convertToDoubleLink2(BinaryTreeNode currentRoot){
		if(currentRoot == null){
			return null;
		}
		BinaryTreeNode leftTail = null;
		if(currentRoot.left != null){
			leftTail = this.convertToDoubleLink2(currentRoot.left);
		}
		if(leftTail != null){
			leftTail.right = currentRoot;
			currentRoot.left = leftTail;
		}
		
		BinaryTreeNode rightTail = null;
		if(currentRoot.right != null){
			rightTail = this.convertToDoubleLink2(currentRoot.right);
		}
		if(rightTail != null){
			BinaryTreeNode rightHead = rightTail;
			while(rightHead.left != null){
				rightHead = rightHead.left;
			}
			rightHead.left = currentRoot;
			currentRoot.right = rightHead;
		}
		return rightTail != null? rightTail : currentRoot;
}

(三)返回双向链表的头结点或者尾结点

// return the head or tail by asLeft
// if left tree return tail otherwise return head
BinaryTreeNode convertToDoubleLink(BinaryTreeNode t, boolean asLeft){
		
		if(t == null){
			return null;
		}
		// t not equal to null
		BinaryTreeNode pLeft = null;
		BinaryTreeNode pRight = null;
		if(t.left != null){
			pLeft = this.convertToDoubleLink(t.left, true);
		}
		if(pLeft != null){
			pLeft.right = t;
			t.left = pLeft;
		}
		if(t.right != null){
			pRight = this.convertToDoubleLink(t.right, false);
		}
		if(pRight != null){
			pRight.left = t;
			t.right = pRight;
		}
		
		BinaryTreeNode returnNode = t;
		if(asLeft){
			while(returnNode.right != null){
				returnNode = returnNode.right;
			}
		}else{
			while(returnNode.left != null){
				returnNode = returnNode.left;
			}
		}
		return returnNode;
}