update

medium/442.find-all-duplicates-in-an-array.go

@@ -0,0 +1,18 @@
+func findDuplicates(nums []int) (res []int) {
+	n := len(nums)
+	for i := 0; i < n; i++ {
+		if idx := abs(nums[i]) - 1; nums[idx] < 0 {
+			res = append(res, idx+1) // Twice
+		} else {
+			nums[idx] = -nums[idx] // Once
+		}
+	}
+	return res
+}
+
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}