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Separate the Digits in an Array

EasyUpdated: Aug 2, 2025
Practice on:
LeetCode

Problem

Given an array of positive integers nums, return an arrayanswer that consists of the digits of each integer innums _after separating them inthe same order they appear in _nums.

To separate the digits of an integer is to get all the digits it has in the same order.

  • For example, for the integer 10921, the separation of its digits is [1,0,9,2,1].

Examples

Example 1

Input: nums = [13,25,83,77]
Output: [1,3,2,5,8,3,7,7]
Explanation: 
- The separation of 13 is [1,3].
- The separation of 25 is [2,5].
- The separation of 83 is [8,3].
- The separation of 77 is [7,7].
answer = [1,3,2,5,8,3,7,7]. Note that answer contains the separations in the same order.

Example 2

Input: nums = [7,1,3,9]
Output: [7,1,3,9]
Explanation: The separation of each integer in nums is itself.
answer = [7,1,3,9].

Constraints

  • 1 <= nums.length <= 1000
  • 1 <= nums[i] <= 10^5

Solution

Method 1 – String Conversion and Flattening

Intuition

We can convert each number to a string, split it into digits, and collect all digits in order. This is direct and efficient for the given constraints.

Approach

  1. For each number in nums, convert it to a string.
  2. For each character in the string, convert it back to an integer and add to the result list.
  3. Return the result list.

Code

C++
class Solution {
public:
    vector<int> separateDigits(vector<int>& nums) {
        vector<int> ans;
        for (int x : nums) {
            string s = to_string(x);
            for (char c : s) ans.push_back(c - '0');
        }
        return ans;
    }
};
Go
func separateDigits(nums []int) []int {
    res := []int{}
    for _, x := range nums {
        for _, c := range fmt.Sprint(x) {
            res = append(res, int(c-'0'))
        }
    }
    return res
}
Java
import java.util.*;
class Solution {
    public List<Integer> separateDigits(int[] nums) {
        List<Integer> ans = new ArrayList<>();
        for (int x : nums) {
            for (char c : String.valueOf(x).toCharArray()) ans.add(c - '0');
        }
        return ans;
    }
}
Kotlin
class Solution {
    fun separateDigits(nums: IntArray): List<Int> {
        val ans = mutableListOf<Int>()
        for (x in nums) {
            for (c in x.toString()) ans.add(c - '0')
        }
        return ans
    }
}
Python
class Solution:
    def separateDigits(self, nums: list[int]) -> list[int]:
        ans = []
        for x in nums:
            ans.extend(int(c) for c in str(x))
        return ans
Rust
impl Solution {
    pub fn separate_digits(nums: Vec<i32>) -> Vec<i32> {
        let mut ans = vec![];
        for x in nums {
            for c in x.to_string().chars() {
                ans.push(c.to_digit(10).unwrap() as i32);
            }
        }
        ans
    }
}
TypeScript
class Solution {
    separateDigits(nums: number[]): number[] {
        const ans: number[] = [];
        for (const x of nums) {
            for (const c of x.toString()) ans.push(Number(c));
        }
        return ans;
    }
}

Complexity

  • ⏰ Time complexity: O(n * d), where n = length of nums, d = max number of digits per number (≤ 5). Each digit is processed once.
  • 🧺 Space complexity: O(n * d), for the output list.

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