401. Binary Watch

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problem

solution

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class Solution {
public:
    int count(int n){
        int ret = 0;
        while(n){
            n &= (n-1);
            ret++;
        }
        return ret;
    }
    vector<string> readBinaryWatch(int turnedOn) {
        vector<string> ret;
        for(int h = 0;h<12;++h){
            for(int m = 0;m<60 ;++m){
                int account = (h<<6)+m;
                if(count(account) == turnedOn){
                    string cand  = to_string(h);
                    if(m<10) cand +=":0";
                    else cand += ":";
                    cand += to_string(m);
                    ret.push_back(cand);           
                }
            }
        }
        return ret;      
    }
};

analysis

  • time complexity O(60*12)
  • space complexity O(n)