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787. K 站中转内最便宜的航班

题目描述

有 n 个城市通过一些航班连接。给你一个数组 flights ，其中 flights[i] = [fromi, toi, pricei] ，表示该航班都从城市 fromi 开始，以价格 pricei 抵达 toi。

现在给定所有的城市和航班，以及出发城市 src 和目的地 dst，你的任务是找到出一条最多经过 k 站中转的路线，使得从 src 到 dst 的 价格最便宜 ，并返回该价格。 如果不存在这样的路线，则输出 -1。

 

示例 1：

输入: 
n = 3, edges = [[0,1,100],[1,2,100],[0,2,500]]
src = 0, dst = 2, k = 1
输出: 200
解释: 
城市航班图如下


从城市 0 到城市 2 在 1 站中转以内的最便宜价格是 200，如图中红色所示。

示例 2：

输入: 
n = 3, edges = [[0,1,100],[1,2,100],[0,2,500]]
src = 0, dst = 2, k = 0
输出: 500
解释: 
城市航班图如下


从城市 0 到城市 2 在 0 站中转以内的最便宜价格是 500，如图中蓝色所示。

 

提示：

  • 1 <= n <= 100
  • 0 <= flights.length <= (n * (n - 1) / 2)
  • flights[i].length == 3
  • 0 <= fromi, toi < n
  • fromi != toi
  • 1 <= pricei <= 104
  • 航班没有重复，且不存在自环
  • 0 <= src, dst, k < n
  • src != dst

解法

方法一：Bellman Ford 算法

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class Solution:
    def findCheapestPrice(
        self, n: int, flights: List[List[int]], src: int, dst: int, k: int
    ) -> int:
        INF = 0x3F3F3F3F
        dist = [INF] * n
        dist[src] = 0
        for _ in range(k + 1):
            backup = dist.copy()
            for f, t, p in flights:
                dist[t] = min(dist[t], backup[f] + p)
        return -1 if dist[dst] == INF else dist[dst]

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class Solution {
    private static final int INF = 0x3f3f3f3f;

    public int findCheapestPrice(int n, int[][] flights, int src, int dst, int k) {
        int[] dist = new int[n];
        int[] backup = new int[n];
        Arrays.fill(dist, INF);
        dist[src] = 0;
        for (int i = 0; i < k + 1; ++i) {
            System.arraycopy(dist, 0, backup, 0, n);
            for (int[] e : flights) {
                int f = e[0], t = e[1], p = e[2];
                dist[t] = Math.min(dist[t], backup[f] + p);
            }
        }
        return dist[dst] == INF ? -1 : dist[dst];
    }
}

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class Solution {
public:
    int findCheapestPrice(int n, vector<vector<int>>& flights, int src, int dst, int k) {
        const int inf = 0x3f3f3f3f;
        vector<int> dist(n, inf);
        vector<int> backup;
        dist[src] = 0;
        for (int i = 0; i < k + 1; ++i) {
            backup = dist;
            for (auto& e : flights) {
                int f = e[0], t = e[1], p = e[2];
                dist[t] = min(dist[t], backup[f] + p);
            }
        }
        return dist[dst] == inf ? -1 : dist[dst];
    }
};

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func findCheapestPrice(n int, flights [][]int, src int, dst int, k int) int {
    const inf = 0x3f3f3f3f
    dist := make([]int, n)
    backup := make([]int, n)
    for i := range dist {
        dist[i] = inf
    }
    dist[src] = 0
    for i := 0; i < k+1; i++ {
        copy(backup, dist)
        for _, e := range flights {
            f, t, p := e[0], e[1], e[2]
            dist[t] = min(dist[t], backup[f]+p)
        }
    }
    if dist[dst] == inf {
        return -1
    }
    return dist[dst]
}

方法二：DFS + 记忆化搜索

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class Solution:
    def findCheapestPrice(
        self, n: int, flights: List[List[int]], src: int, dst: int, k: int
    ) -> int:
        @cache
        def dfs(u, k):
            if u == dst:
                return 0
            if k <= 0:
                return inf
            k -= 1
            ans = inf
            for v, p in g[u]:
                ans = min(ans, dfs(v, k) + p)
            return ans

        g = defaultdict(list)
        for u, v, p in flights:
            g[u].append((v, p))
        ans = dfs(src, k + 1)
        return -1 if ans >= inf else ans

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class Solution {
    private int[][] memo;
    private int[][] g;
    private int dst;
    private static final int INF = (int) 1e6;

    public int findCheapestPrice(int n, int[][] flights, int src, int dst, int k) {
        n += 10;
        memo = new int[n][n];
        for (int i = 0; i < n; ++i) {
            Arrays.fill(memo[i], -1);
        }
        g = new int[n][n];
        for (int[] e : flights) {
            g[e[0]][e[1]] = e[2];
        }
        this.dst = dst;
        int ans = dfs(src, k + 1);
        return ans >= INF ? -1 : ans;
    }

    private int dfs(int u, int k) {
        if (memo[u][k] != -1) {
            return memo[u][k];
        }
        if (u == dst) {
            return 0;
        }
        if (k <= 0) {
            return INF;
        }
        int ans = INF;
        for (int v = 0; v < g[u].length; ++v) {
            if (g[u][v] > 0) {
                ans = Math.min(ans, dfs(v, k - 1) + g[u][v]);
            }
        }
        memo[u][k] = ans;
        return ans;
    }
}

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class Solution {
public:
    vector<vector<int>> memo;
    vector<vector<int>> g;
    int dst;
    int inf = 1e6;

    int findCheapestPrice(int n, vector<vector<int>>& flights, int src, int dst, int k) {
        n += 10;
        memo.resize(n, vector<int>(n, -1));
        g.resize(n, vector<int>(n));
        for (auto& e : flights) g[e[0]][e[1]] = e[2];
        this->dst = dst;
        int ans = dfs(src, k + 1);
        return ans >= inf ? -1 : ans;
    }

    int dfs(int u, int k) {
        if (memo[u][k] != -1) return memo[u][k];
        if (u == dst) return 0;
        if (k <= 0) return inf;
        int ans = inf;
        for (int v = 0; v < g[u].size(); ++v)
            if (g[u][v] > 0)
                ans = min(ans, dfs(v, k - 1) + g[u][v]);
        memo[u][k] = ans;
        return memo[u][k];
    }
};

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func findCheapestPrice(n int, flights [][]int, src int, dst int, k int) int {
    n += 10
    memo := make([][]int, n)
    g := make([][]int, n)
    for i := range memo {
        memo[i] = make([]int, n)
        g[i] = make([]int, n)
        for j := range memo[i] {
            memo[i][j] = -1
        }
    }

    for _, e := range flights {
        g[e[0]][e[1]] = e[2]
    }
    inf := int(1e6)
    var dfs func(u, k int) int
    dfs = func(u, k int) int {
        if memo[u][k] != -1 {
            return memo[u][k]
        }
        if u == dst {
            return 0
        }
        if k <= 0 {
            return inf
        }
        ans := inf
        for v, p := range g[u] {
            if p > 0 {
                ans = min(ans, dfs(v, k-1)+p)
            }
        }
        memo[u][k] = ans
        return ans
    }
    ans := dfs(src, k+1)
    if ans >= inf {
        return -1
    }
    return ans
}

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