2222_NumberofWaystoSelectBuildings - a920604a/leetcode GitHub Wiki

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title: 2222. Number of Ways to Select Buildings tags:
- Math categories: leetcode comments: false

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

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)