lintcode,将二叉树拆成链表

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将一棵二叉树按照前序遍历拆解成为一个假链表。所谓的假链表是说，用二叉树的 right 指针，来表示链表中的 next 指针。

一刷ac
解题思路：可以用一个栈进行前序遍历，前序遍历过程中将left置为空，right置为栈顶元素。

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: a TreeNode, the root of the binary tree     * @return: nothing     */    public void flatten(TreeNode root) {        if(root == null) return;        Stack<TreeNode> stack = new Stack<TreeNode>();        stack.push(root);        while(!stack.empty()){            TreeNode node = stack.pop();            if(node.right != null) stack.push(node.right);            if(node.left != null) stack.push(node.left);            if(stack.empty()){                node.left = null;                node.right = null;            }else{                node.left = null;                node.right = stack.peek();            }        }    }}

也可以递归

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */public class Solution {    /**     * @param root: a TreeNode, the root of the binary tree     * @return: nothing     */    public TreeNode lastnode = null;    public void flatten(TreeNode root) {        if(root == null) return;        if(lastnode != null){            lastnode.left = null;            lastnode.right = root;        }        lastnode = root;        TreeNode right = root.right;        flatten(root.left);        flatten(right);    }}

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