add 20 exercises

data/1.C语言初阶/2.数据类型/1.变量/config.json

+{
+  "node_id": "569d5e11c4fc5de7844053d9a733c5e8",
+  "keywords": [
+    "变量",
+    "C语言"
+  ],
+  "children": [],
+  "export": [
+    {
+      "file": "variables.c",
+      "variants": "variables.json",
+      "depends": []
+    }
+  ]
+}
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data/1.C语言初阶/2.数据类型/1.变量/variables.c

+#include <stdio.h>
+
+int main()
+{
+    int num = 1;
+    int max_num = 10;
+    int step = 1;
+    int result = 0;
+    
+    while (num <= max_num)
+    {
+        result = num * num;
+        printf("%d ^ 2 = %d\n", num, result);
+        num = num + step;
+    }
+
+    return 0;
+}
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data/1.C语言初阶/2.数据类型/1.变量/variables.json

+{
+    "multiline": [
+        {
+            "int step = 1;": "int step = 2;"
+        },
+        {
+            "while (num <= max_num)": "while (num < max_num)"
+        },
+        {
+            "num = num + step;": "num + step;"
+        }
+    ]
+}
\ No newline at end of file

data/1.C语言初阶/2.数据类型/1.变量/variables.md

+# 变量：求正整数的平方
+
+使用while循环修改变量值，依次打印10以内(包含10)所有正整数的平方。请从以下选项中找出正确答案。
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