1615. Maximal Network Rank

1615. Maximal Network Rank (Medium)

There is an infrastructure of n cities with some number of roads connecting these cities. Each roads[i] = [ai, bi] indicates that there is a bidirectional road between cities ai and bi.

The network rank of two different cities is defined as the total number of directly connected roads to either city. If a road is directly connected to both cities, it is only counted once.

The maximal network rank of the infrastructure is the maximum network rank of all pairs of different cities.

Given the integer n and the array roads, return the maximal network rank of the entire infrastructure.

 

Example 1:

Input: n = 4, roads = [[0,1],[0,3],[1,2],[1,3]]
Output: 4
Explanation: The network rank of cities 0 and 1 is 4 as there are 4 roads that are connected to either 0 or 1. The road between 0 and 1 is only counted once.

Example 2:

Input: n = 5, roads = [[0,1],[0,3],[1,2],[1,3],[2,3],[2,4]]
Output: 5
Explanation: There are 5 roads that are connected to cities 1 or 2.

Example 3:

Input: n = 8, roads = [[0,1],[1,2],[2,3],[2,4],[5,6],[5,7]]
Output: 5
Explanation: The network rank of 2 and 5 is 5. Notice that all the cities do not have to be connected.

 

Constraints:

• 2 <= n <= 100
• 0 <= roads.length <= n * (n - 1) / 2
• roads[i].length == 2
• 0 <= ai, bi <= n-1
• ai != bi
• Each pair of cities has at most one road connecting them.

Related Topics:
Graph

Solution 1.

// OJ: https://leetcode.com/problems/maximal-network-rank/
// Author: github.com/lzl124631x
// Time: O(N^2)
// Space: O(N^2)
class Solution {
public:
    int maximalNetworkRank(int n, vector<vector<int>>& A) {
        vector<vector<int>> G(n, vector<int>(n));
        vector<int> degree(n);
        for (auto &r : A) {
            int u = r[0], v = r[1];
            G[u][v] = G[v][u] = 1;
            degree[u]++;
            degree[v]++;
        }
        int ans = 0;
        for (int i = 0; i < n; ++i) {
            for (int j = i + 1; j < n; ++j) {
                ans = max(ans, degree[i] + degree[j] - G[i][j]);
            }
        }
        return ans;
    }
};

Solution 2.

Let deg[i] be the degree of node i. For node u and v, the answer is deg[u] + deg[v] if they are not connected, and deg[u] + dev[v] - 1 otherwise.

Let first be the greatest degree, second be the second greatest degree (first and second can be the same if there are multiple nodes with the largest degree). We just need to consider the cities with first and second degrees because:

• The result of selecting from such nodes is at least first + second - 1 (when they are connected).
• If we select a city with degree x < second, then the result is at best first + x < first + second, i.e. first + x <= first + second - 1, which is no better than selecting from first and second

So, selecting from first and second must be optimal.

Let x be the number of cities with first degree. So there are ${x \choose 2 } = \frac{x(x-1)}{2}$ pairs among these cities. Let m be the number of roads.

• If x = 1, then we must pick a city from the cities with second degree. Enumerating such cities takes O(n) time.
• If x > 1 and ${x\choose 2} > m$, then the roads don't cover all the possible connections between those first degree cities, so there must be a pair of first degree cities unconnected. Picking them results in first * 2.
• IF x > 1 and ${x\choose 2} <= m$, then we don't need to consider second degree city because its result first + second must be no better than the result of picking two first degree cities, first + first - 1. We just need to enumerate all the city pairs among first degree cities, which takes O(x * (x-1) / 2) = O(m) time.

So overall this solution takes O(n + m) time.

// OJ: https://leetcode.com/problems/maximal-network-rank/
// Author: github.com/lzl124631x
// Time: O(N + M)
// Space: O(N + M)
// Ref: https://leetcode-cn.com/problems/maximal-network-rank/solution/onm-mei-ju-fa-by-zerotrac2/
class Solution {
public:
    int maximalNetworkRank(int n, vector<vector<int>>& A) {
        if (A.empty()) return 0;
        vector<unordered_set<int>> G(n);
        vector<int> deg(n), firstNodes, secondNodes;
        for (auto &e : A) {
            int u = e[0], v = e[1];
            G[u].insert(v);
            G[v].insert(u);
            deg[u]++;
            deg[v]++;
        }
        int first = 0, second = 0, m = A.size();
        for (int d : deg) {
            if (d > first) {
                second = first;
                first = d;
            } else if (d > second) second = d;
        }
        for (int i = 0; i < n; ++i) {
            if (deg[i] == first) firstNodes.push_back(i);
            else if (deg[i] == second) secondNodes.push_back(i);
        }
        if (firstNodes.size() == 1) {
            int u = firstNodes[0];
            for (int v : secondNodes) {
                if (G[u].count(v) == 0) return first + second;
            }
            return first + second - 1;
        } else {
            if (firstNodes.size() * (firstNodes.size() - 1) / 2 > m) return first * 2;
            for (int i = 0; i < firstNodes.size(); ++i) {
                for (int j = i + 1; j < firstNodes.size(); ++j) {
                    if (G[firstNodes[i]].count(firstNodes[j]) == 0) return first * 2;
                }
            }
            return first * 2 - 1;
        }
    }
};

Solution 3.

// OJ: https://leetcode.com/problems/maximal-network-rank/
// Author: github.com/lzl124631x
// Time: O(N + M)
// Space: O(N)
// Ref: https://leetcode.com/problems/maximal-network-rank/discuss/889206/Java-2ms-O(m)
class Solution {
public:
    int maximalNetworkRank(int n, vector<vector<int>>& A) {
        if (A.empty()) return 0;
        vector<int> deg(n);
        for (auto &e : A) {
            int u = e[0], v = e[1];
            deg[u]++;
            deg[v]++;
        }
        int first = 0, second = 0, firstCnt = 0, secondCnt = 0, cnt = 0;
        for (int d : deg) {
            if (d > first) {
                second = first;
                first = d;
            } else if (d == first) continue;
            else if (d > second) second = d;
        }
        for (int d : deg) {
            if (d == first) ++firstCnt;
            else if (d == second) ++secondCnt;
        }
        if (firstCnt == 1) {
            for (auto &e : A) {
                int u = e[0], v = e[1];
                cnt += (deg[u] == first && deg[v] == second) || (deg[u] == second && deg[v] == first);
            }
            return first + second - (secondCnt <= cnt);
        } else {
            for (auto &e : A) {
                int u = e[0], v = e[1];
                cnt += deg[u] == first && deg[v] == first;
            }
            return first * 2 - (firstCnt * (firstCnt - 1) / 2 <= cnt);
        }
    }
};