原题链接: https://oj.leetcode.com/problems/binary-tree-level-order-traversal/

public List<List<Integer>> levelOrder(TreeNode root) {
    if (root == null) return new ArrayList<List<Integer>>();

    List<List<Integer>> result = new ArrayList<List<Integer>>();
    List<Integer> row = new ArrayList<Integer>();

    row.add(root.val);
    result.add(row);

    row = new ArrayList<Integer>();

    List<List<Integer>> leftList = null;
    if (root.left != null) {
        row.add(root.left.val);
        leftList = levelOrder(root.left);
    }

    List<List<Integer>> rightList = null;
    if (root.right != null) {
        row.add(root.right.val);
        rightList = levelOrder(root.right);
    }

    if (row.size() > 0) {
        result.add(row);
    }

    if (leftList != null) {
        leftList = leftList.subList(1, leftList.size());
    }

    if (rightList != null) {
        rightList = rightList.subList(1, rightList.size());
    }

    int i = 0;
    int leftSize = leftList != null ? leftList.size() : 0;
    int rightSize = rightList != null ? rightList.size() : 0;

    for (; i < leftSize; i++) {
        row = new ArrayList<Integer>();

        row.addAll(leftList.get(i));
        if (i < rightSize) row.addAll(rightList.get(i)); 

        result.add(row);
    }
    for (; i < rightSize; i++) {
        row = new ArrayList<Integer>();

        row.addAll(rightList.get(i));

        result.add(row);
    }

    return result;
}