2222. Number of Ways to Select Buildings

發表於 | 分類於

problem

solution

need four variable , reprsentation

  • the number of visited zero
  • the number of unvisited zero
  • the number of visited one
  • the number of unvisited one
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
class Solution {
public:
    long long numberOfWays(string s) {
        int unused_zeros = 0, unused_ones  = 0, n = s.size();
        for(char c:s) unused_ones+=(c=='1');
        unused_zeros = n - unused_ones;
        long long ret = 0;
        int used_zeros =0, used_ones = 0;
        for(char c:s){
            if(c == '1'){
                // 010
                ret+=unused_zeros*used_zeros;
                used_ones++;
                unused_ones--;  
            }
            else{
                //101
                ret+=unused_ones*used_ones;
                used_zeros++;
                unused_zeros--;                   
            }
        }
        return ret;
    }
};

analysis

  • time complexity O(n)
  • space complexity O(1)