题目

21根火柴游戏。现有21根火柴，两人轮流取，每人每次可以取1至4根，不可多取（假如多取或者取走的数量不在合法的范围内，则要求重新输入），也不能不取，谁取最后一根火柴谁输。请编写一个程序进行人机对弈，要求人先取，计算机后取；请设计一种计算机取走火柴的规则，使得计算机一方为常胜将军。
**输入格式要求：“%d” 提示信息：“Game begin:\n” “How many sticks do you wish to take (1~%d)?”
**输出格式要求：" %d sticks left in the pile.\n" " You have taken the last sticks.\n"
" ***You lose!\nGame Over.\n" “Computer take %d sticks.\n”
程序运行示例如下：
Game begin:
How many sticks do you wish to take (1~4)?6
How many sticks do you wish to take (1~4)?3
18 sticks left in the pile.
Computer take 2 sticks.
16 sticks left in the pile.
How many sticks do you wish to take (1~4)?3
13 sticks left in the pile.
Computer take 2 sticks.
11 sticks left in the pile.
How many sticks do you wish to take (1~4)?3
8 sticks left in the pile.
Computer take 2 sticks.
6 sticks left in the pile.
How many sticks do you wish to take (1~4)?3
3 sticks left in the pile.
Computer take 2 sticks.
1 sticks left in the pile.
How many sticks do you wish to take (1~1)?2
How many sticks do you wish to take (1~1)?1
You have taken the last sticks.
***You lose!
Game Over.

实现

思路：电脑为后手，其输入只需和人类输入之和凑够5即可，经过4轮之后，最后一个自然落在了人类手上。

#include 
#define N 4
int main(){printf("Game begin:\n");int left = 21;while(left > 0){int numA, numB;int len = left > N ? N : left;printf("How many sticks do you wish to take (1~%d)?", len);scanf("%d", &numA);while(numA < 1 || numA > len){printf("How many sticks do you wish to take (1~%d)?", len);scanf("%d", &numA);}left -= numA;if(left > 0){printf(" %d sticks left in the pile.\n", left);}if(left > 1){switch (numA) {case 1:numB = 4;break;case 2:numB = 3;break;case 3:numB = 2;break;case 4:numB = 1;break;}left -= numB;printf("Computer take %d sticks.\n", numB);printf(" %d sticks left in the pile.\n", left);fflush(stdout);} else{   //left == 1left--;printf("You have taken the last sticks.\n");printf(" ***You lose!\nGame Over.\n");}}
}