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package class_2022_02_3_week;
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.HashMap;
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import java.util.Map.Entry;
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// 测试链接 : https://leetcode.com/problems/cheapest-flights-within-k-stops/
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public class Code01_CheapestFlightsWithinKStops {
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// 类似宽度优先遍历
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// n，城市数量，0...n-1
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// flights = { {0,7,100}, {15,3,300} .... }
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// src -> 源点
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// dst -> 目标点
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// k -> 最多能中转几次
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public static int findCheapestPrice1(int n, int[][] flights, int src, int dst, int k) {
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// 图
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// 0 : {1, 500} {7, 20} {13, 70}
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ArrayList<ArrayList<int[]>> graph = new ArrayList<>();
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for (int i = 0; i < n; i++) {
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graph.add(new ArrayList<int[]>());
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}
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for (int[] line : flights) {
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// [0, 7 , 200]
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// 0 : { {7, 200} }
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graph.get(line[0]).add(new int[] { line[1], line[2] });
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}
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// distance[i] : 从源点src出发的情况下，到达i号点，最少费用是多少
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int[] cost = new int[n];
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Arrays.fill(cost, Integer.MAX_VALUE);
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cost[src] = 0;
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HashMap<Integer, Integer> curMap = new HashMap<>();
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curMap.put(src, 0);
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for (int i = 0; i <= k; i++) {
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HashMap<Integer, Integer> nextMap = new HashMap<>();
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for (Entry<Integer, Integer> entry : curMap.entrySet()) {
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// A -> from 100
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// from -> ????
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int from = entry.getKey();
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int preCost = entry.getValue();
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for (int[] line : graph.get(from)) {
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int to = line[0];
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int curCost = line[1];
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cost[to] = Math.min(cost[to], preCost + curCost);
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nextMap.put(to,
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Math.min(nextMap.getOrDefault(to, Integer.MAX_VALUE),
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preCost + curCost));
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}
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}
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curMap = nextMap;
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}
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return cost[dst] == Integer.MAX_VALUE ? -1 : cost[dst];
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}
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// Bellman Ford
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// 快！
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public static int findCheapestPrice2(int n, int[][] flights, int src, int dst, int k) {
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int[] cost = new int[n];
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Arrays.fill(cost, Integer.MAX_VALUE);
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cost[src] = 0;
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for (int i = 0; i <= k; i++) {
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int[] next = Arrays.copyOf(cost, n);
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for (int[] f : flights) {
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if (cost[f[0]] != Integer.MAX_VALUE) {
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next[f[1]] = Math.min(next[f[1]], cost[f[0]] + f[2]);
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}
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}
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cost = next;
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}
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return cost[dst] == Integer.MAX_VALUE ? -1 : cost[dst];
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}
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}
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