Adding Spaces to a String - Practice Coding Problems

Adding Spaces to a String - Problem

You are given a 0-indexed string s and a 0-indexed integer array spaces that describes the indices in the original string where spaces will be added. Each space should be inserted before the character at the given index.

For example, given s = "EnjoyYourCoffee" and spaces = [5, 9], we place spaces before 'Y' and 'C', which are at indices 5 and 9 respectively. Thus, we obtain "Enjoy Your Coffee".

Return the modified string after the spaces have been added.

Input & Output

Example 1 — Basic Case

$ Input: s = "EnjoyYourCoffee", spaces = [5,9]

› Output: "Enjoy Your Coffee"

💡 Note: Insert space before index 5 (character 'Y') and index 9 (character 'C') to get "Enjoy Your Coffee"

Example 2 — Multiple Spaces

$ Input: s = "icodeinpython", spaces = [1,5,7,9]

› Output: "i code in py thon"

💡 Note: Insert spaces at positions 1, 5, 7, and 9 to separate the string into readable segments

Example 3 — Single Space

$ Input: s = "spacing", spaces = [0]

› Output: " spacing"

💡 Note: Insert space at the beginning (index 0) to get " spacing"

Constraints

1 ≤ s.length ≤ 3 × 10⁴
s consists only of lowercase and uppercase English letters.
1 ≤ spaces.length ≤ 3 × 10⁴
0 ≤ spaces[i] ≤ s.length - 1
All the values of spaces are strictly increasing.

Visualization

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Understanding the Visualization

Input Analysis

Original string and array of space positions

Space Insertion

Insert spaces before characters at given indices

Output

Modified string with spaces added

Key Takeaway

🎯 Key Insight: Use two pointers to traverse the string and space positions simultaneously for efficient single-pass construction

Asked in

G Google 15 a Amazon 12 M Microsoft 8

The key insight is to use two pointers to track the original string position and space array index simultaneously. The optimal approach builds the result in a single pass by checking if the current position matches a space position and inserting a space before adding the character. Time: O(n + m), Space: O(n + m).

Common Approaches

Approach	Time	Space	Notes
✓ Two Pointers with Pre-allocation	O(n + m)	O(n + m)	Pre-allocate result array and use two pointers to efficiently build the final string
String Concatenation	O(n²)	O(n)	Build result by concatenating substrings with spaces inserted at correct positions

Two Pointers with Pre-allocation — Algorithm Steps

Calculate final string length
Initialize result array
Use two pointers to fill result array
Convert to string

Visualization

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Step-by-Step Walkthrough

Initialize Pointers

String pointer at 0, space pointer at 0

Process Characters

Add space when position matches, then add character

Build Result

Efficiently construct final string in one pass

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* solution(char* s, int* spaces, int spacesSize) {
    int len = strlen(s);
    char* result = malloc((len + spacesSize + 1) * sizeof(char));
    int resultIdx = 0;
    int spaceIdx = 0;
    
    for (int i = 0; i < len; i++) {
        if (spaceIdx < spacesSize && i == spaces[spaceIdx]) {
            result[resultIdx++] = ' ';
            spaceIdx++;
        }
        result[resultIdx++] = s[i];
    }
    
    result[resultIdx] = '\0';
    return result;
}

int main() {
    char s[1001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;
    
    char line[10001];
    fgets(line, sizeof(line), stdin);
    
    // Parse array
    int spaces[1000];
    int spacesSize = 0;
    char* token = strtok(line + 1, ",]");
    while (token != NULL) {
        spaces[spacesSize++] = atoi(token);
        token = strtok(NULL, ",]");
    }
    
    char* result = solution(s, spaces, spacesSize);
    printf("%s\n", result);
    free(result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n + m)

Single pass through string (n characters) and spaces array (m elements)

✓ Linear Growth

Space Complexity

O(n + m)

Result array stores final string with added spaces

⚡ Linearithmic Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Two Pointers with Pre-allocation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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