LeetCode 2714. Find Shortest Path with K Hops Solution in Java, C++, Python & Go | Explanation + Code

Description

You are given a positive integer n which is the number of nodes of a 0-indexed undirected weighted connected graph and a 0-indexed 2D array edges where edges[i] = [ui, vi, wi] indicates that there is an edge between nodes ui and vi with weight wi.

You are also given two nodes s and d, and a positive integer k, your task is to find the shortest path from s to d, but you can hop over at most k edges. In other words, make the weight of at most k edges 0 and then find the shortest path from s to d.

Return the length of the shortest path from s to d with the given condition.

 

Example 1:

Input: n = 4, edges = [[0,1,4],[0,2,2],[2,3,6]], s = 1, d = 3, k = 2
Output: 2
Explanation: In this example there is only one path from node 1 (the green node) to node 3 (the red node), which is (1->0->2->3) and the length of it is 4 + 2 + 6 = 12. Now we can make weight of two edges 0, we make weight of the blue edges 0, then we have 0 + 2 + 0 = 2. It can be shown that 2 is the minimum length of a path we can achieve with the given condition.

Example 2:

Input: n = 7, edges = [[3,1,9],[3,2,4],[4,0,9],[0,5,6],[3,6,2],[6,0,4],[1,2,4]], s = 4, d = 1, k = 2
Output: 6
Explanation: In this example there are 2 paths from node 4 (the green node) to node 1 (the red node), which are (4->0->6->3->2->1) and (4->0->6->3->1). The first one has the length 9 + 4 + 2 + 4 + 4 = 23, and the second one has the length 9 + 4 + 2 + 9 = 24. Now if we make weight of the blue edges 0, we get the shortest path with the length 0 + 4 + 2 + 0 = 6. It can be shown that 6 is the minimum length of a path we can achieve with the given condition.

Example 3:

Input: n = 5, edges = [[0,4,2],[0,1,3],[0,2,1],[2,1,4],[1,3,4],[3,4,7]], s = 2, d = 3, k = 1
Output: 3
Explanation: In this example there are 4 paths from node 2 (the green node) to node 3 (the red node), which are (2->1->3), (2->0->1->3), (2->1->0->4->3) and (2->0->4->3). The first two have the length 4 + 4 = 1 + 3 + 4 = 8, the third one has the length 4 + 3 + 2 + 7 = 16 and the last one has the length 1 + 2 + 7 = 10. Now if we make weight of the blue edge 0, we get the shortest path with the length 1 + 2 + 0 = 3. It can be shown that 3 is the minimum length of a path we can achieve with the given condition.

 

Constraints:

• 2 <= n <= 500
• n - 1 <= edges.length <= min(104, n * (n - 1) / 2)
• edges[i].length = 3
• 0 <= edges[i][0], edges[i][1] <= n - 1
• 1 <= edges[i][2] <= 106
• 0 <= s, d, k <= n - 1
• s != d
• The input is generated such that the graph is connected and has no repeated edges or self-loops

Solutions

Solution 1: Dijkstra Algorithm

First, we construct a graph g based on the given edges, where g[u] represents all neighboring nodes of node u and their corresponding edge weights.

Then, we use Dijkstra's algorithm to find the shortest path from node s to node d. However, we need to make some modifications to Dijkstra's algorithm:

• We need to record the shortest path length from each node u to node d, but since we can cross at most k edges, we need to record the shortest path length from each node u to node d and the number of edges crossed t, i.e., dist[u][t] represents the shortest path length from node u to node d and the number of edges crossed is t.
• We need to use a priority queue to maintain the current shortest path, but since we need to record the number of edges crossed, we need to use a triple (dis, u, t) to represent the current shortest path, where dis represents the current shortest path length, and u and t represent the current node and the number of edges crossed, respectively.

Finally, we only need to return the minimum value in dist[d][0..k].

The time complexity is O(n² × log n), and the space complexity is O(n × k), where n represents the number of nodes and k represents the maximum number of edges crossed.

PythonJavaC++Go

class Solution: def shortestPathWithHops( self, n: int, edges: List[List[int]], s: int, d: int, k: int ) -> int: g = [[] for _ in range(n)] for u, v, w in edges: g[u].append((v, w)) g[v].append((u, w)) dist = [[inf] * (k + 1) for _ in range(n)] dist[s][0] = 0 pq = [(0, s, 0)] while pq: dis, u, t = heappop(pq) for v, w in g[u]: if t + 1 <= k and dist[v][t + 1] > dis: dist[v][t + 1] = dis heappush(pq, (dis, v, t + 1)) if dist[v][t] > dis + w: dist[v][t] = dis + w heappush(pq, (dis + w, v, t)) return int(min(dist[d]))(code-box)

class Solution { public int shortestPathWithHops(int n, int[][] edges, int s, int d, int k) { List<int[]>[] g = new List[n]; Arrays.setAll(g, i -> new ArrayList<>()); for (int[] e : edges) { int u = e[0], v = e[1], w = e[2]; g[u].add(new int[] {v, w}); g[v].add(new int[] {u, w}); } PriorityQueue<int[]> pq = new PriorityQueue<>((a, b) -> a[0] - b[0]); pq.offer(new int[] {0, s, 0}); int[][] dist = new int[n][k + 1]; final int inf = 1 << 30; for (int[] e : dist) { Arrays.fill(e, inf); } dist[s][0] = 0; while (!pq.isEmpty()) { int[] p = pq.poll(); int dis = p[0], u = p[1], t = p[2]; for (int[] e : g[u]) { int v = e[0], w = e[1]; if (t + 1 <= k && dist[v][t + 1] > dis) { dist[v][t + 1] = dis; pq.offer(new int[] {dis, v, t + 1}); } if (dist[v][t] > dis + w) { dist[v][t] = dis + w; pq.offer(new int[] {dis + w, v, t}); } } } int ans = inf; for (int i = 0; i <= k; ++i) { ans = Math.min(ans, dist[d][i]); } return ans; } }(code-box)

class Solution { public: int shortestPathWithHops(int n, vector<vector<int>>& edges, int s, int d, int k) { vector<pair<int, int>> g[n]; for (auto& e : edges) { int u = e[0], v = e[1], w = e[2]; g[u].emplace_back(v, w); g[v].emplace_back(u, w); } priority_queue<tuple<int, int, int>, vector<tuple<int, int, int>>, greater<tuple<int, int, int>>> pq; pq.emplace(0, s, 0); int dist[n][k + 1]; memset(dist, 0x3f, sizeof(dist)); dist[s][0] = 0; while (!pq.empty()) { auto [dis, u, t] = pq.top(); pq.pop(); for (auto [v, w] : g[u]) { if (t + 1 <= k && dist[v][t + 1] > dis) { dist[v][t + 1] = dis; pq.emplace(dis, v, t + 1); } if (dist[v][t] > dis + w) { dist[v][t] = dis + w; pq.emplace(dis + w, v, t); } } } return *min_element(dist[d], dist[d] + k + 1); } };(code-box)

func shortestPathWithHops(n int, edges [][]int, s int, d int, k int) int { g := make([][][2]int, n) for _, e := range edges { u, v, w := e[0], e[1], e[2] g[u] = append(g[u], [2]int{v, w}) g[v] = append(g[v], [2]int{u, w}) } pq := hp{{0, s, 0}} dist := make([][]int, n) for i := range dist { dist[i] = make([]int, k+1) for j := range dist[i] { dist[i][j] = math.MaxInt32 } } dist[s][0] = 0 for len(pq) > 0 { p := heap.Pop(&pq).(tuple) dis, u, t := p.dis, p.u, p.t for _, e := range g[u] { v, w := e[0], e[1] if t+1 <= k && dist[v][t+1] > dis { dist[v][t+1] = dis heap.Push(&pq, tuple{dis, v, t + 1}) } if dist[v][t] > dis+w { dist[v][t] = dis + w heap.Push(&pq, tuple{dis + w, v, t}) } } } return slices.Min(dist[d]) } type tuple struct{ dis, u, t int } type hp []tuple func (h hp) Len() int { return len(h) } func (h hp) Less(i, j int) bool { return h[i].dis < h[j].dis } func (h hp) Swap(i, j int) { h[i], h[j] = h[j], h[i] } func (h *hp) Push(v any) { *h = append(*h, v.(tuple)) } func (h *hp) Pop() any { a := *h; v := a[len(a)-1]; *h = a[:len(a)-1]; return v }(code-box)