feat: add C code in chapter_greedy (#755)

* add fractional_knapsack.c

* update CMakeLists

* add c code in chapter_greedy

* fix header format

* format code by clang-format

* remove extra comments

* replace ternary operator to MACRO

* parameters form adjustment

* Update fractional_knapsack.c

* Update max_capacity.c

* Update max_product_cutting.c

* add comments for consistency with cpp ver

* move MIN&MAX macro to source file

* typo fix

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/c/chapter_greedy/fractional_knapsack.c

@@ -0,0 +1,62 @@
+/**
+ * File: fractional_knapsack.c
+ * Created Time: 2023-09-14
+ * Author: xianii (xianyi.xia@outlook.com)
+ */
+
+#include "../utils/common.h"
+
+/* 物品 */
+struct item {
+    int w; // 物品重量
+    int v; // 物品价值
+};
+
+typedef struct item Item;
+
+/* 按照价值密度排序 */
+int sortByValueDensity(const void *a, const void *b) {
+    Item *t1 = (Item *)a;
+    Item *t2 = (Item *)b;
+    return (float)(t1->v) / t1->w < (float)(t2->v) / t2->w;
+}
+
+/* 分数背包：贪心 */
+float fractionalKnapsack(int wgt[], int val[], int itemCount, int cap) {
+    // 创建物品列表，包含两个属性：重量、价值
+    Item *items = malloc(sizeof(Item) * itemCount);
+    for (int i = 0; i < itemCount; i++) {
+        items[i] = (Item){.w = wgt[i], .v = val[i]};
+    }
+    // 按照单位价值 item.v / item.w 从高到低进行排序
+    qsort(items, (size_t)itemCount, sizeof(Item), sortByValueDensity);
+    // 循环贪心选择
+    float res = 0.0;
+    for (int i = 0; i < itemCount; i++) {
+        if (items[i].w <= cap) {
+            // 若剩余容量充足，则将当前物品整个装进背包
+            res += items[i].v;
+            cap -= items[i].w;
+        } else {
+            // 若剩余容量不足，则将当前物品的一部分装进背包
+            res += (float)cap / items[i].w * items[i].v;
+            cap = 0;
+            break;
+        }
+    }
+    free(items);
+    return res;
+}
+
+/* Driver Code */
+int main(void) {
+    int wgt[] = {10, 20, 30, 40, 50};
+    int val[] = {50, 120, 150, 210, 240};
+    int capacity = 50;
+
+    // 贪心算法
+    float res = fractionalKnapsack(wgt, val, sizeof(wgt) / sizeof(int), capacity);
+    printf("不超过背包容量的最大物品价值为 %0.2f\n", res);
+
+    return 0;
+}