4. Median of Two Sorted Arrays

Pure math. Median definition is the median is used for dividing a set into two equal length subsets, that one subset is always greater than the other.

102. Binary Tree Level Order Traversal

  • The best way to to iterate tree level by level is to use for loop in Java. (BFS)
  • return null is not same as return; (empty)
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public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        
        if (root == null) return res;
        
        q.offer(root);
        while (!q.isEmpty()) {
            Integer levelLength = q.size();
            List<Integer> list = new LinkedList<Integer>();
            
            for (int i = 0; i < levelLength; i++) {
                TreeNode node = q.poll();
                list.add(node.val);
                if (node.left != null) q.offer(node.left);
                if (node.right != null) q.offer(node.right);
            }
            res.add(list);
        }
        
        return res;
    }
}
// recursive
public class Solution1 {
    List<List<Integer>> res = new ArrayList<List<Integer>>();
        
    public List<List<Integer>> levelOrder(TreeNode root) {
        helper(root, 0);
        return res;
    }
    
    private void helper(TreeNode root, Integer depth) {
        if (root == null) return;
        if (res.size() == depth) res.add(new LinkedList<Integer>());
        
        res.get(depth).add(root.val);
        helper(root.left, depth+1);
        helper(root.right, depth+1);
    }
}