C#: faster than 90%
Intuition\nUse list.AddRange method.\n\n# Approach\nUse list.AddRange method to add digits of each number.\n\n# Complexity\n- Time complexity:\nO(n)\n\n- Space 
# Intuition\nUse list.AddRange method.\n\n# Approach\nUse list.AddRange method to add digits of each number.\n\n# Complexity\n- Time complexity:\n$$O(n)$$\n\n- Space complexity:\n$$O(n)$$\n\n# Code\n```\npublic class Solution {\n    public int[] SeparateDigits(int[] nums) {\n        var list = new List<int>();\n        for (int i = 0; i < nums.Length; i++)\n        {\n            list.AddRange(GetAllDigits(nums[i]));\n        }\n        return list.ToArray();\n    }\n    private List<int> GetAllDigits(int n)\n    {\n        var rs = new List<int>();\n        while (n > 0)\n        {\n            rs.Insert(0, n % 10);\n            n /= 10;\n        }\n        return rs;\n    }\n}\n```
['C#']