double dist(double x1, double y1, double x2, double y2)
{
	return sqrt((x1 -x2)*(x1 - x2) + (y1- y2)*(y1 - y2));
}

#include <stdio.h>
#include <math.h>
typedef struct point { double x; double y; } point;
double dist(point a, point b)
{
	return hypot(a.x - b.x, a.y - b.y);  //求直角三角形斜边长，此时看起来清爽多了
}
int main()
{
	point A, B;
	A.x = A.y = 1;B.x = B.y = 2;
	printf("%.3f\n", dist(A, B));
	return 0;
}

#include <stdio.h>
#include <math.h>
long long C(int n, int m){
	if(m < n-m) m = n - m;
	long long ans = 1;
	for(int i = m+1; i <= n; i++) ans *= i;
	for(int i = 1; i <= n-m; i++) ans /= i;
	return ans;
}
int main()
{
	int m,n;
	while(scanf("%d%d", &m, &n)!=EOF && n!=0)
		printf("%ld\n", C(n, m));
	return 0;
}

#include <stdio.h>
#include <math.h>
int is_prime(int n){
	if(n <= 1) return 0;
	int m = floor(sqrt(double(n) + 0.5)); //进行四舍五入避免了浮点误差
	for(int i = 2; i <=m; i++)
		if(n % i == 0) return 0;
	return 1;
}
int main()
{
	int n;
	while(scanf("%d", &n) != EOF)
		printf("%d\n", is_prime(n));
	return 0;
}

void swap(int *a, int *b){
	int *t;
	*t = *a; *a = *b; *b = *t;
}

#include <iostream>
int sum1(int *a, int n){           //方法一
	int ans = 0;
	for(int i = 0; i < n; i++)
		ans += a[i];
	return ans;
}
int sum2(int *begin, int *end){   //方法二
	int n = end - begin;
	int ans = 0;
	for(int i = 0; i < n; i++)
		ans += begin[i];
	return ans;
}
int sum3(int *begin, int *end){   //方法三
	int ans = 0;
	for(int *p = begin; p != end; p++)
		ans += *p;
	return ans;
}
int main(){
	int b[5] = {1, 3, 5, 7, 9};
	std::cout << sum1(b+1, 4) << '\n' << sum2(b+1, b+5) << '\n' << sum3(b+1, b+5) << std::endl;
	return 0;
}

#include <cstdio>
#include <cstring>
#include <cstdlib>
int cmp(const void *a, const void *b){  //qsort中当cmp分别返回负数，0和正数时，分别表示a<b,a==b,a>b
	return *(int *)a - *(int *)b;       //这里的新内容是指向常数的万能指针const void*，可进行强制类型转换，转换为int型指针
}
int main(){
	char str1[105], str2[105];
	int cnt1[26], cnt2[26];
	memset(cnt1, 0, sizeof(cnt1));
	memset(cnt2, 0, sizeof(cnt2));
	while(scanf("%s%s", str1, str2) !=EOF){
		for(int i = 0; i < strlen(str1); i++){
			cnt1[str1[i] - 'A']++;
			cnt2[str2[i] - 'A']++;
		}
		qsort(cnt1, 26, sizeof(int), cmp);
		qsort(cnt2, 26, sizeof(int), cmp);
		int tag = 1;
		for(int i = 0; i < 26; i++)
			if(cnt1[i] != cnt2[i]) tag = 0;
		if(tag) printf("YES\n");
		else printf("NO\n");
	}
	return 0;
}

(gdb) bt
#0 f (n = 0) at factorial.c： 4
#1 0x00401308 in f (n = 1) at factorial.c: 3
#2 0x00401308 in f (n = 2) at factorial.c: 3
#3 0x00401308 in f (n = 3) at factorial.c: 3
#4 0x00401359 in main   () at factorial.c: 6

#include <cstdio>
#include <cstring>
char word[105],gue[105];
bool win, lose;
int left, chance, rnd; //rnd表示轮数，当输入-1时结束
void guess(char ch);
int main(){
	while(scanf("%d%s%s", &rnd, word, gue) != EOF && rnd != -1){
		printf("Round %d\n", rnd);
		chance = 7;
		left = strlen(word);
		win = lose = false;
		for(int i = 0; i < strlen(gue); i++){
			guess(gue[i]);
			if(lose || win) break;
		}
		if(win) printf("You Win.\n");
		else if(lose) printf("You Lose.\n");
		else printf("You chickened out.\n");
	}
	return 0;
}
	
void guess(char ch){
	int bad = 1;
	for(int i = 0; i < strlen(word); i++){
		if(ch == word[i]){ left--; word[i] = ' '; bad = 0; }
	}
	if(bad) chance--;
	if(!left && chance) win = true;
	if(!chance && !left) lose = true;
}

<span style="font-size:10px;">#include <cstdio>
int go(int p, int q, int t);
int num[30];
int n, k, m, left;
int main(){
	while(scanf("%d%d%d", &n, &k, &m) != EOF){
		left = n;
		int p1 = n, p2 = 1;
		for(int i = 1; i <= n; i++) num[i] = i;
		while(left){
			p1 = go(p1, 1, k);
			p2 = go(p2, -1, m);
			printf("%3d", p1); left--;
			if(p2 != p1) { printf("%3d", p2); left--; }
			num[p1] = num[p2] = 0;
			if(left) printf(",");
		}
		printf("\n");
	}
	return 0;
}

int go(int p, int q, int t){
	while(t--){
		do{ p = (p - 1 + q + n) % n + 1; }while(num[p] == 0); //p-1使标号由1~n变为0~n-1，再按照常规方式寻找p-1的下一位置，最后将标号加1由0~n-1还原至1~n
	}                         //走到下一个非0的数字
	return p;
}</span>

<span style="font-size:10px;">#include<stdio.h>
#include<string.h>

int code[8][1<<8];
int readchar(){
  for(;;) {
    int ch = getchar();
    if(ch != '\n' && ch != '\r') return ch;
  }
}

int readint(int c){    //有些编码文本很长，可能会由多行组成，此函数是方便读取编码文本的
  int v = 0;
  while(c--) v = v * 2 + readchar() - '0';
  return v;
}

int readcodes(){
  memset(code, 0, sizeof(code));
  code[1][0] = readchar();   //直接调到下一行开始读取，先用readchar读取第一个字符再用普通的getchar读取剩余的字符直到遇到\n，防止不必要的错误
  for(int len = 2; len <= 7; len++) {
    for(int i = 0; i < (1<<len)-1; i++) {
      int ch = getchar();     
      if(ch == EOF) return 0;
      if(ch == '\n' || ch == '\r') return 1; //\n 换行,将当前位置移到下一行开头,\r 回车,将当前位置移到本行开头
      code[len][i] = ch;
    }
  }
  return 1;
}

int main() {
  while(readcodes()) {
    for(;;) {
      int len = readint(3);   //先读取字节头，来查看本节编码长度
      if(len == 0) break;
      for(;;) {
        int v = readint(len); //读取本节的编码，至遇到全1结束
        if(v == (1 << len)-1) break;
        putchar(code[len][v]);
      }
    }
    putchar('\n');
  }
  return 0;
}</span>