算法题练习
课程表
public boolean canFinish(int n, int[][] p) {
// 1. 准备⼯作
int[] in = new int[n]; // 统计每⼀个顶点的⼊度
Map<Integer, List<Integer>> edges = new HashMap<>(); // 邻接表存图
// 2. 建图
for (int i = 0; i < p.length; i++) {
int a = p[i][0], b = p[i][1]; // b -> a
if (!edges.containsKey(b)) {
edges.put(b, new ArrayList<>());
}
edges.get(b).add(a);
in[a]++;
}
// 3. 拓扑排序
Queue<Integer> q = new LinkedList<>();
// (1) 先把⼊度为 0 的点，加⼊到队列中
for (int i = 0; i < n; i++) {
if (in[i] == 0)
q.add(i);
}
// (2) bfs
while (!q.isEmpty()) {
int t = q.poll();
for (int a : edges.getOrDefault(t, new ArrayList<>())) {
in[a]--;
if (in[a] == 0)
q.add(a);
}
}
// 4. 判断是否有环
for (int x : in)
if (x != 0)
return false;
return true;
}
课程表2(就多了一个统计的过程)
public int[] findOrder(int n, int[][] p) {
// 1. 准备⼯作
int[] in = new int[n]; // 统计每⼀个顶点的⼊度
Map<Integer, List<Integer>> edges = new HashMap<>(); // 邻接表存图
// 2. 建图
for (int i = 0; i < p.length; i++) {
int a = p[i][0], b = p[i][1]; // b -> a
if (!edges.containsKey(b)) {
edges.put(b, new ArrayList<>());
}
edges.get(b).add(a);
in[a]++;
}
// 3. 拓扑排序
Queue<Integer> q = new LinkedList<>();
int[] ret = new int[n];
int index = 0;
// (1) 先把⼊度为 0 的点，加⼊到队列中
for (int i = 0; i < n; i++) {
if (in[i] == 0)
q.add(i);
}
// (2) bfs
while (!q.isEmpty()) {
int t = q.poll();
ret[index++] = t;
for (int a : edges.getOrDefault(t, new ArrayList<>())) {
in[a]--;
if (in[a] == 0)
q.add(a);
}
}
// 4. 判断是否有环
if(index ==n)
return ret;
return new int[0];
}
Map<Character, Set<Character>> edges = new HashMap<>(); // 邻接表
Map<Character, Integer> in = new HashMap<>(); // 统计每个节点的⼊度
boolean check;
public String alienOrder(String[] words) {
// 1. 初始化⼊度哈希表 + 建图
for (String s : words) {
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
in.put(ch, 0);
}
}
int n = words.length;
for (int i = 0; i < n; i++) {
for (int j = i + 1; j < n; j++) {
add(words[i], words[j]);
if (check == true)
return "";
}
}
// 2. 拓扑排序
Queue<Character> q = new LinkedList<>();
for (char ch : in.keySet()) {
if (in.get(ch) == 0)
q.add(ch);
}
StringBuffer ret = new StringBuffer();
while (!q.isEmpty()) {
char t = q.poll();
ret.append(t);
for (char ch : edges.getOrDefault(t,new HashSet<>())) {
in.put(ch, in.get(ch) - 1);
if (in.get(ch) == 0)
q.add(ch);
}
}
// 3. 判断
for (char ch : in.keySet()) {
if (in.get(ch) != 0)
return "";
}
return ret.toString();
}
public void add(String s1, String s2) {
int n = Math.min(s1.length(), s2.length());
int i = 0;
for (; i < n; i++) {
char c1 = s1.charAt(i), c2 = s2.charAt(i);
if (c1 != c2) {
if (!edges.containsKey(c1)) {
edges.put(c1, new HashSet<>());
}
if (!edges.get(c1).contains(c2)) {
edges.get(c1).add(c2);
in.put(c2, in.get(c2) + 1);
}
break;
}
}
if (i == s2.length() && i < s1.length())
check = true;
}