103. Binary Tree Zigzag Level Order Traversal

# Medium, BFS

Most are as same as #102. Set a flag to judge odd or even layer. Once it's even layer, reverse the level then append to result.

Java(DFS)

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

Java(BFS)

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null) return res;
        
        List<Integer> level = new ArrayList<Integer>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        int even = 1;
        queue.add(root);
        
        while(!queue.isEmpty()) {
            level = new ArrayList<Integer>();
            int l = queue.size();
            while(l > 0) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if(node.left != null) queue.add(node.left);
                if(node.right != null) queue.add(node.right);
                l --;
            }
            if(even == -1) 
                Collections.reverse(level);
            res.add(level);
            even *= (-1);
        }
        
        return res;
    }
}