Day-22 Binary Tree Zigzag Level Order Traversal

/**
 * 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>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res=new ArrayList<>();
        if(root==null) return res;
        boolean reverse=false;
        Queue<TreeNode> st=new LinkedList<>();
        st.add(root);
        
        while(!st.isEmpty()){
            int size=st.size();
            List<Integer> l=new ArrayList<>();
            while(size-->0){
                TreeNode temp=st.remove();
                l.add(temp.val);
                if(temp.left!=null){
                    st.add(temp.left);
                }
                if(temp.right!=null){
                    st.add(temp.right);
                }
            }
            
            if(reverse){
                Collections.reverse(l);
            }
            reverse=!reverse;
            res.add(l);
        }
        
        return res;
    }
}

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res=new ArrayList<>();
        if(root==null) return res;
        helper(root,0,res);
        return res;
    }
    
    void helper(TreeNode node,int level,List<List<Integer>> res){
        
        if(node==null){
            return;
        }
        
        if(res.size()==level){
            res.add(new ArrayList<>());
        }
        
        if(level%2==0){
            res.get(level).add(node.val);
        }else{
            res.get(level).add(0,node.val);
        }
        
        helper(node.left,level+1,res);
        helper(node.right,level+1,res);
        
    }
}