No102.binary-tree-level-order-traversal.cs

/*
 * Difficulty:
 * Medium
 *
 * Desc:
 * Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
 *
 * For example:
 * Given binary tree [3,9,20,null,null,15,7],
 *    3
 *   / \
 *  9  20
 *    /  \
 *   15   7
 * return its level order traversal as:
 * [
 *  [3],
 *  [9,20],
 *  [15,7]
 * ]
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public IList<IList<int>> LevelOrder(TreeNode root) {
        List<IList<int>> res = new List<IList<int>>();
        if (root == null) return res;

        Queue<TreeNode> q = new Queue<TreeNode>();
        q.Enqueue(root);

        while (q.Count > 0) {
            int len = q.Count;
            List<int> layer = new List<int>();
            while (len > 0) {
                TreeNode node = q.Dequeue();
                layer.Add(node.val);
                if (node.left != null) q.Enqueue(node.left);
                if (node.right != null) q.Enqueue(node.right);
                len -= 1;
            }
            res.Add(layer);
        }
        return res;
    }
}