方法一 哈希表
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
List<TreeNode> list = new ArrayList<>();
public void flatten(TreeNode root) {
help(root);
TreeNode temp = new TreeNode(-1);
for(int i = 0; i < list.size(); i++) {
temp.right = list.get(i);
temp = temp.right;
temp.left = null;
}
}
public void help(TreeNode root) {
if(root == null) {
return;
}
list.add(root);
help(root.left);
help(root.right);
}
}
方法二 迭代法
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public void flatten(TreeNode root) {
if(root == null) {
return;
}
TreeNode pre = null;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while(!stack.isEmpty()) {
TreeNode cur = stack.pop();
if(pre != null) {
//用来保存前一个节点
pre.left = null;
pre.right = cur;
}
if(cur.right != null) {
stack.push(cur.right);
}
if(cur.left != null) {
stack.push(cur.left);
}
pre = cur;
}
}
}