Add kotlin code block for array.md and backtracking_algorithm.md. (krahets#1185)

Author: curtishd

docs/chapter_array_and_linkedlist/array.md

@@ -108,7 +108,9 @@
 === "Kotlin"
 
     ```kotlin title="array.kt"
-
+    /* 初始化数组 */
+    var arr = IntArray(5) // { 0, 0, 0, 0, 0 }
+    var nums = intArrayOf(1, 3, 2, 5, 4)
     ```
 
 === "Zig"

docs/chapter_backtracking/backtracking_algorithm.md

@@ -380,7 +380,27 @@
 === "Kotlin"
 
     ```kotlin title=""
-
+    /* 回溯算法框架 */
+    fun backtrack(state: State?, choices: List<Choice?>, res: List<State?>?) {
+        // 判断是否为解
+        if (isSolution(state)) {
+            // 记录解
+            recordSolution(state, res)
+            // 不再继续搜索
+            return
+        }
+        // 遍历所有选择
+        for (choice in choices) {
+            // 剪枝：判断选择是否合法
+            if (isValid(state, choice)) {
+                // 尝试：做出选择，更新状态
+                makeChoice(state, choice)
+                backtrack(state, choices, res)
+                // 回退：撤销选择，恢复到之前的状态
+                undoChoice(state, choice)
+            }
+        }
+    }
     ```
 
 === "Zig"