计算二叉树每层的和

    private static class BinaryNode {
        BinaryNode left;
        BinaryNode right;
        int val;

        private BinaryNode(BinaryNode left, BinaryNode right, int val) {
            this.left = left;
            this.right = right;
            this.val = val;
        }
    }


    private static void levelSum(BinaryNode root, Map<Integer, Integer> result, int level) {
        if (root == null) {
            return;
        }
        int levelSum;
        if (result.containsKey(level)) {
            levelSum = result.get(level);
        } else {
            levelSum = 0;
        }

        levelSum += root.val;
        result.put(level, levelSum);

        levelSum(root.left, result, level + 1);
        levelSum(root.right, result, level + 1);
    }

    public static void main(String args[]) {
        BinaryNode level2Left1 = new BinaryNode(null, null, 4);
        BinaryNode level2Right1 = new BinaryNode(null, null, 5);
        BinaryNode level1Left = new BinaryNode(level2Left1, level2Right1, 2);

        BinaryNode level2Left2 = new BinaryNode(null, null, 1);
        BinaryNode level2Right2 = new BinaryNode(null, null, 2);
        BinaryNode level1Right = new BinaryNode(level2Left2, level2Right2, 3);

        BinaryNode root = new BinaryNode(level1Left, level1Right, 1);
        Map<Integer, Integer> result = new HashMap<Integer, Integer>();
        levelSum(root, result, 0);
        System.out.println(result.values());
    }