LeetCode401. 二进制手表

class Solution {
    private List<String> res = new LinkedList<>();
    public List<String> readBinaryWatch(int num) {
        dfs(num, 0, 0, 1, 1, new LinkedList<Integer>(), new LinkedList<Integer>());
        return res;
    }

    // h：小时数，m：分钟数
    // hstart、mstart：从hstart、mstart开始继续遍历小时和分钟
    // hours：选中的小时，minutes：选中的分钟
    private void dfs(int num, int h, int m, int hstart, int mstart, List<Integer> hours, List<Integer> minutes) {
        if (hours.size() + minutes.size() == num) {
            if (h < 12 && m < 60) res.add(String.format("%d:%02d", h, m));
            return;
        }
        for (int i = hstart; i <= 8; i <<= 1) {
            hours.addLast(i);
            dfs(num, h + i, m, i << 1, mstart, hours, minutes);
            hours.removeLast();
        }
        for (int i = mstart; i <= 32; i <<= 1) {
            minutes.addLast(i);
            dfs(num, h, m + i, 9, i << 1, hours, minutes); // hstart 直接设置为9（> 8)，避免重复计算
            minutes.removeLast();
        }
    }
}