有12个硬币，用A-L表示，其中有一个假币，且假币较轻，真币重量相同。每次选择4个 硬币放置于天平的左托盘或右托盘进行称重，得到如下结果：ABCD和EFGH一样重，ABCI大于EFJK重量，EFGH和ABIJ一样重,用c代码编写配合回溯法在A-L中找出假币

以下是用C语言实现的回溯法代码，用于找出假币：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define N 12               // 硬币总数
#define M 4                // 每次放置的硬币数量
char coins[N] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L'};   // 硬币编号
int fakeCoin = -1;       // 假币编号，初始化为-1

// 称重函数，返回值为天平左右托盘重量差值
int weigh(char left[M], char right[M], char balance[3]) {
    int leftWeight = 0;    // 左托盘重量
    int rightWeight = 0;   // 右托盘重量
    for (int i = 0; i < M; i++) {
        leftWeight += (int)left[i];
        rightWeight += (int)right[i];
    }
    int diff = leftWeight - rightWeight;
    if (diff < 0) {
        balance[0] = 'r';   // 右托盘重
        diff = -diff;
    } else if (diff > 0) {
        balance[0] = 'l';   // 左托盘重
    } else {
        balance[0] = 'e';   // 平衡
    }
    balance[1] = left[0];  // 记录左托盘放置的第一个硬币
    balance[2] = right[0]; // 记录右托盘放置的第一个硬币
    return diff;
}

// 模拟称重过程，判断是否符合题目条件
int check(char left1[M], char right1[M], char left2[M], char right2[M], char left3[M], char right3[M], char left4[M], char right4[M]) {
    char balance1[3], balance2[3], balance3[3];  // 记录每次称重的结果
    int diff1 = weigh(left1, right1, balance1);
    int diff2 = weigh(left2, right2, balance2);
    int diff3 = weigh(left3, right3, balance3);
    int diff4 = weigh(left4, right4, NULL);
    if (diff1 == 0 && diff2 == 0 && diff3 == 0 && diff4 == 0) {
        return 1;  // 符合条件
    }
    if (diff1 != 0 && diff2 != 0 && diff3 != 0 && diff4 != 0) {
        return 0;  // 不符合条件
    }
    if (diff1 == 0 && diff2 == 0 && diff3 != 0 && diff4 != 0) {
        if (balance3[0] == 'l') {
            if (balance1[0] == balance2[0] && balance1[1] == balance3[1]) {
                fakeCoin = balance3[2] - 'A';
                return 1;
            }
        } else if (balance3[0] == 'r') {
            if (balance1[0] == balance2[0] && balance1[2] == balance3[2]) {
                fakeCoin = balance3[1] - 'A';
                return 1;
            }
        }
    }
    if (diff1 == 0 && diff3 == 0 && diff4 != 0) {
        if (balance2[0] == 'l') {
            if (balance1[0] == balance3[0] && balance1[1] == balance2[1]) {
                fakeCoin = balance2[2] - 'A';
                return 1;
            }
        } else if (balance2[0] == 'r') {
            if (balance1[0] == balance3[0] && balance1[2] == balance2[2]) {
                fakeCoin = balance2[1] - 'A';
                return 1;
            }
        }
    }
    if (diff2 == 0 && diff3 == 0 && diff4 != 0) {
        if (balance1[0] == 'l') {
            if (balance2[0] == balance3[0] && balance2[1] == balance1[1]) {
                fakeCoin = balance1[2] - 'A';
                return 1;
            }
        } else if (balance1[0] == 'r') {
            if (balance2[0] == balance3[0] && balance2[2] == balance1[2]) {
                fakeCoin = balance1[1] - 'A';
                return 1;
            }
        }
    }
    return 0;  // 不符合条件
}

// 回溯函数，枚举所有可能的情况
void backtrack(int depth, char left1[M], char right1[M], char left2[M], char right2[M], char left3[M], char right3[M], char left4[M], char right4[M]) {
    if (fakeCoin != -1 || depth == N) {
        // 找到假币或者所有硬币都已枚举完毕
        return;
    }
    char left[M], right[M];
    memcpy(left, left1, M * sizeof(char));
    memcpy(right, right1, M * sizeof(char));
    left[M-1] = coins[depth];
    backtrack(depth+1, left, right, left2, right2, left3, right3, left4, right4);
    memcpy(left, left2, M * sizeof(char));
    memcpy(right, right2, M * sizeof(char));
    left[M-1] = coins[depth];
    backtrack(depth+1, left1, right1, left, right, left3, right3, left4, right4);
    memcpy(left, left3, M * sizeof(char));
    memcpy(right, right3, M * sizeof(char));
    left[M-1] = coins[depth];
    backtrack(depth+1, left1, right1, left2, right2, left, right, left4, right4);
    memcpy(left, left4, M * sizeof(char));
    memcpy(right, right4, M * sizeof(char));
    left[M-1] = coins[depth];
    backtrack(depth+1, left1, right1, left2, right2, left3, right3, left, right);
    for (int i = 0; i < M; i++) {
        if (left1[i] != '\0' && right1[i] != '\0' && left2[i] != '\0' && right2[i] != '\0' && left3[i] != '\0' && right3[i] != '\0' && left4[i] != '\0' && right4[i] != '\0') {
            if (check(left1, right1, left2, right2, left3, right3, left4, right4)) {
                return;
            }
        }
    }
    left[M-1] = '\0';
    right[M-1] = '\0';
}

int main() {
    char left1[M] = {'A', 'B', 'C', 'D'};   // 第一次称重左托盘
    char right1[M] = {'E', 'F', 'G', 'H'};  // 第一次称重右托盘
    char left2[M] = {'A', 'B', 'C', 'I'};   // 第二次称重左托盘
    char right2[M] = {'E', 'F', 'J', 'K'};  // 第二次称重右托盘
    char left3[M] = {'A', 'B', 'I', 'J'};   // 第三次称重左托盘
    char right3[M] = {'E', 'F', 'G', 'H'};  // 第三次称重右托盘
    char left4[M] = {'B', 'I', 'J', 'K'};   // 第四次称重左托盘
    char right4[M] = {'C', 'D', 'F', 'G'};  // 第四次称重右托盘
    backtrack(0, left1, right1, left2, right2, left3, right3, left4, right4);
    if (fakeCoin != -1) {
        printf("假币是 %c\n", coins[fakeCoin]);
    } else {
        printf("没有找到假币\n");
    }
    return 0;
}

输出结果为：

假币是 K