feat: add question 78

Author: yinxin630

78.子集.js

@@ -0,0 +1,27 @@
+/*
+ * @lc app=leetcode.cn id=78 lang=javascript
+ *
+ * [78] 子集
+ */
+
+// @lc code=start
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var subsets = function(nums) {
+    const result = [];
+    const n = 2 ** nums.length;
+    for (let i = 0; i < n; i++) {
+        const t = [];
+        const str = i.toString(2);
+        for (let j = 0; j < nums.length; j++) {
+            if (str[j] === '1') {
+                t.push(nums[str.length - 1 - j]);
+            }
+        }
+        result.push(t);
+    }
+    return result;
+};
+// @lc code=end