[leetcode]107. Binary Tree Level Order Traversal II

Solution 1:DFS+Collections.reverse(res);

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res=new ArrayList<>();
        create(root,res,1);
        Collections.reverse(res);
        return res;
        
    }
    
    public void create(TreeNode node,List<List<Integer>>lists,int level){
       if(node==null)return;
        if(lists.size()<level){
            List<Integer> list=new ArrayList<>();
            list.add(node.val);
            lists.add(list);
        }else{
            List<Integer> list=lists.get(level-1);
            list.add(node.val);
        }
    
        create(node.left,lists,level+1);
        create(node.right,lists,level+1);
    }
    
}

Solution 2:BFS+STACK

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> lists=new ArrayList<List<Integer>>();
        if(root==null)return lists;
        
        Stack<List<Integer>> stack = new Stack<>();
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        queue.offer(root);
        
        while(!queue.isEmpty()){
            List<Integer> list=new ArrayList<Integer>();
            int size=queue.size();
            for(int i=0;i<size;i++){
                TreeNode temp=queue.poll();
                list.add(temp.val);
                if(temp.left!=null)queue.offer(temp.left);
                if(temp.right!=null)queue.offer(temp.right);
            }
            stack.add(list);
        }
        while(!stack.isEmpty()){
            lists.add(stack.pop());
        }
        return lists;
    }
    
    
}