小红书8.28笔试题解（java选手）
后端
代码题难度一般，都是经典题。太久没刷题了，我做的还是稍微慢了点，3道题一共写了40多分钟。
A1
//经典排序public static void main(String[] args) { Scanner in = new Scanner(System.in); int n = in.nextInt(); int m=in.nextInt(); int id=in.nextInt(); long arr[]=new long[n]; for (int i = 0; i < n; i++) { for (int j = 0; j < m; j++) { arr[i]+=in.nextInt(); } arr[i]= (long) (arr[i]*1e6-i-1); } Arrays.sort(arr); int ans=-1; for (int i = 0; i < n ; i++) { if((arr[i]+id)%1000000==0)ans=n-i; } System.out.println(ans); }
A2
//排序+双指针
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int n = in.nextInt();
long k = in.nextLong();
long arr[] = new long[n];
for (int i = 0; i < n; i++) {
arr[i] = in.nextInt();
}
long ans = 0;
Arrays.sort(arr);
int j = n - 1;
for (int i = 0; i < n; i++) {
while (j >= 0&&arr[i] * arr[j] >= k ) {
j--;
}
if (i > j) {
ans--;
}
ans += n -1- j;
}
System.out.println(ans);
} A3
//DFS+贪心
static int ans=0;
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int n = in.nextInt();
ArrayList arr[]=new ArrayList[n+1];
for (int i = 0; i <=n ; i++) {
arr[i]=new ArrayList();
}
for (int i = 0; i < n-1; i++) {
int idx=in.nextInt();
arr[i+2].add(idx);
arr[idx].add(i+2);
}
DFS(-1,1,arr,false);
System.out.println(ans);
}
static boolean DFS(int father,int now ,ArrayList arr[],boolean can){
boolean b=true;
for (int i = 0; i < arr[now].size(); i++) {
int val= (int) arr[now].get(i);
if(father==val)continue;
b=b&!DFS(now ,val,arr,b);
}
if(can&&b)ans++;
return can&&b;
} 
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