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package class25;
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// 本题测试链接 : https://leetcode.com/problems/gas-station/
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// 注意本题的实现比leetcode上的问法更加通用
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// leetcode只让返回其中一个良好出发点的位置
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// 本题是返回结果数组，每一个出发点是否是良好出发点都求出来了
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// 得到结果数组的过程，时间复杂度O(N)，额外空间复杂度O(1)
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public class Code04_GasStation {
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public static int canCompleteCircuit(int[] gas, int[] cost) {
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if (gas == null || gas.length == 0) {
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return -1;
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}
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if (gas.length == 1) {
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return gas[0] < cost[0] ? -1 : 0;
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}
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boolean[] good = stations(cost, gas);
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for (int i = 0; i < gas.length; i++) {
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if (good[i]) {
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return i;
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}
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}
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return -1;
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}
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public static boolean[] stations(int[] cost, int[] gas) {
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if (cost == null || gas == null || cost.length < 2 || cost.length != gas.length) {
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return null;
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}
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int init = changeDisArrayGetInit(cost, gas);
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return init == -1 ? new boolean[cost.length] : enlargeArea(cost, init);
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}
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public static int changeDisArrayGetInit(int[] dis, int[] oil) {
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int init = -1;
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for (int i = 0; i < dis.length; i++) {
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dis[i] = oil[i] - dis[i];
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if (dis[i] >= 0) {
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init = i;
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}
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}
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return init;
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}
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public static boolean[] enlargeArea(int[] dis, int init) {
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boolean[] res = new boolean[dis.length];
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int start = init;
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int end = nextIndex(init, dis.length);
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int need = 0;
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int rest = 0;
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do {
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// 当前来到的start已经在连通区域中，可以确定后续的开始点一定无法转完一圈
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if (start != init && start == lastIndex(end, dis.length)) {
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break;
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}
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// 当前来到的start不在连通区域中，就扩充连通区域
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// start(5) -> 联通区的头部(7) -> 2
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// start(-2) -> 联通区的头部(7) -> 9
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if (dis[start] < need) { // 当前start无法接到连通区的头部
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need -= dis[start];
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} else { // 当前start可以接到连通区的头部，开始扩充连通区域的尾巴
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// start(7) -> 联通区的头部(5)
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rest += dis[start] - need;
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need = 0;
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while (rest >= 0 && end != start) {
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rest += dis[end];
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end = nextIndex(end, dis.length);
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}
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// 如果连通区域已经覆盖整个环，当前的start是良好出发点，进入2阶段
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if (rest >= 0) {
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res[start] = true;
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connectGood(dis, lastIndex(start, dis.length), init, res);
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break;
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}
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}
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start = lastIndex(start, dis.length);
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} while (start != init);
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return res;
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}
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// 已知start的next方向上有一个良好出发点
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// start如果可以达到这个良好出发点，那么从start出发一定可以转一圈
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public static void connectGood(int[] dis, int start, int init, boolean[] res) {
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int need = 0;
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while (start != init) {
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if (dis[start] < need) {
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need -= dis[start];
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} else {
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res[start] = true;
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need = 0;
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}
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start = lastIndex(start, dis.length);
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}
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}
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public static int lastIndex(int index, int size) {
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return index == 0 ? (size - 1) : index - 1;
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}
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public static int nextIndex(int index, int size) {
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return index == size - 1 ? 0 : (index + 1);
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}
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}
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