Today I have done leetcode's November LeetCoding Challenge with cpp.

November LeetCoding Challenge 12

Description

Bus Routes

You are given an array routes representing bus routes where routes[i] is a bus route that the ith bus repeats forever.

• For example, if routes[0] = [1, 5, 7], this means that the 0th bus travels in the sequence 1 -> 5 -> 7 -> 1 -> 5 -> 7 -> 1 -> ... forever.

You will start at the bus stop source (You are not on any bus initially), and you want to go to the bus stop target. You can travel between bus stops by buses only.

Return the least number of buses you must take to travel from source to target. Return -1 if it is not possible.

Example 1:

Input: routes = [[1,2,7],[3,6,7]], source = 1, target = 6
Output: 2
Explanation: The best strategy is take the first bus to the bus stop 7, then take the second bus to the bus stop 6.

Example 2:

Input: routes = [[7,12],[4,5,15],[6],[15,19],[9,12,13]], source = 15, target = 12
Output: -1

Constraints:

• 1 <= routes.length <= 500.
• 1 <= routes[i].length <= 105
• All the values of routes[i] are unique.
• sum(routes[i].length) <= 105
• 0 <= routes[i][j] < 106
• 0 <= source, target < 106

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
public:
  int numBusesToDestination(vector<vector<int>>& routes, int source, int target) {
    if(source == target) return 0;
    map<int, vector<int>> stopRoutes;
    for(int i = 0; i < routes.size(); ++i) {
      for(auto bus : routes[i]) {
        stopRoutes[bus].push_back(i);
      }
    }

    vector<bool> visitRoutes(routes.size());
    set<int> visitStop;
    queue<int> q;
    for(auto route : stopRoutes[source]) {
      q.push(route);
    }

    int count = 1;
    while(q.size()) {
      int sz = q.size();
      for(int _ = 0; _ < sz; ++_) {
        auto route = q.front();
        q.pop();
        if(visitRoutes[route]) continue;
        visitRoutes[route] = true;

        for(auto stop : routes[route]) {
          if(stop == target) return count;
          if(visitStop.count(stop)) continue;
          for(auto route : stopRoutes[stop]) {
            q.push(route);
          }
          visitStop.insert(stop);
        }
      }
      count += 1;
    }

    return -1;
  }
};

// Accepted
// 49/49 cases passed (293 ms)
// Your runtime beats 60.83 % of cpp submissions
// Your memory usage beats 41.02 % of cpp submissions (72.5 MB)