leetcode中遇到的回朔算法可以解決的問題

401. Binary Watch

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right.

For example, the above binary watch reads "3:25".

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

Input: n = 1
Return: ["1:00", "2:00", "4:00", "8:00", "0:01", "0:02", "0:04", "0:08", "0:16", "0:32"]

code：

class Solution {
public:
    vector<string> readBinaryWatch(int num) {
        vector<string> result;
        int time=0;
        backtrack(num, 0, time, result);
        return result;
    }
private:
    void backtrack(int num, int startBit, int time, vector<string>& result) {
        if (num == 0) {
            addTimeToResult(time, result);
            return;
        }
        for (int i = startBit; i < 10; i++) {
            backtrack(num - 1, i + 1, time | (1 << i), result);  // time | (1 << i)
        }
    }
    void addTimeToResult(int time, vector<string>& result) 
    {
        int m = time & 0x3f, h = time >> 6;
        if (m < 60 && h < 12) 
        {
            string mine;
            string hour=to_string(h);
            string last;
            if(m<10) 
                mine="0"+to_string(m);
            else
                mine=to_string(m);
            last=hour + ":" + mine;
            result.push_back(last);
        }
    }
};