107.二叉树的层序遍历II
/**
* 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 List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> ans = new LinkedList<List<Integer>>();
if(root==null){
return ans;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while(!queue.isEmpty()){
List<Integer> levelList = new ArrayList<>();
int size = queue.size();
for(int i = 0;i<size;i++){
TreeNode now = queue.poll();
levelList.add(now.val);
if(now.left!=null){
queue.offer(now.left);
}
if(now.right!=null){
queue.offer(now.right);
}
}
//遍历完一层节点后，将存储该层节点值得列表添加到结果列表头部
ans.add(0,levelList);
}
return ans;
}
}
/**
* 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 List<List<Integer>> levelOrderBottom(TreeNode root) {
List<List<Integer>> ans = new LinkedList<List<Integer>>();
if(root==null){
return ans;
}
Queue<TreeNode> queue = new LinkedList<>();
Stack<List<Integer>> stack = new Stack<>();
queue.offer(root);
while(!queue.isEmpty()){
List<Integer> levelList = new ArrayList<>();
int size = queue.size();
for(int i = 0;i<size;i++){
TreeNode now = queue.poll();
levelList.add(now.val);
if(now.left!=null){
queue.offer(now.left);
}
if(now.right!=null){
queue.offer(now.right);
}
}
//定义一个栈，每次添加时入栈
stack.push(levelList);
}
//弹出元素
while(!stack.isEmpty()){
ans.add(stack.pop());
}
return ans;
}
}
