Removing Stars From a String

Removing Stars From a String - Problem

You are given a string s, which contains stars *.

In one operation, you can:

Choose a star in s.
Remove the closest non-star character to its left, as well as remove the star itself.

Return the string after all stars have been removed.

Note:

The input will be generated such that the operation is always possible.
It can be shown that the resulting string will always be unique.

Input & Output

Example 1 — Basic Star Removal

$ Input: s = "leet**cod*e"

› Output: lecoe

💡 Note: Process left to right: 'leet' → stack has [l,e,e,t]. First '*' removes 't' → [l,e,e]. Second '*' removes 'e' → [l,e]. Then 'cod' → [l,e,c,o,d]. Another '*' removes 'd' → [l,e,c,o]. Finally 'e' → [l,e,c,o,e] = "lecoe"

Example 2 — All Characters Removed

$ Input: s = "erase*****"

› Output: ""

💡 Note: Start with 'erase' in stack. Five stars remove all five characters one by one, leaving empty string

Example 3 — No Stars

$ Input: s = "hello"

› Output: hello

💡 Note: No stars present, so all characters remain: 'hello'

Constraints

1 ≤ s.length ≤ 10⁵
s consists of lowercase English letters and stars *
The operation is always possible (guaranteed by problem)

Visualization

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Asked in

a Amazon 15 M Microsoft 12 G Google 8

The key insight is treating stars as backspace operations. A stack naturally handles this by pushing characters and popping when encountering stars. Time: O(n), Space: O(n).

Common Approaches

✓ Brute Force String Manipulation

⏱️ Time: O(n²) Space: O(n)

For each star found, locate the nearest non-star character to its left, remove both characters, then repeat until no stars remain. This requires multiple passes through the string.

Stack-Based Solution

⏱️ Time: O(n) Space: O(n)

Process the string left to right. For each character: if it's not a star, push to stack; if it's a star, pop from stack (removing the previous character). The stack contents form the final result.

Brute Force String Manipulation — Algorithm Steps

Find the first star in the string
Find the closest non-star character to its left
Remove both characters
Repeat until no stars remain

Visualization

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Step-by-Step Walkthrough

Find Star

Locate first star in string

Remove Pair

Remove star and character to its left

Repeat

Continue until no stars remain

Code -

solution.c — C

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

char* solution(char* s) {
    int len = strlen(s);
    char* result = malloc((len + 1) * sizeof(char));
    strcpy(result, s);
    
    while (strchr(result, '*') != NULL) {
        char* starPos = strchr(result, '*');
        int index = starPos - result;
        
        if (index > 0) {
            // Remove the character before the star and the star itself
            memmove(&result[index-1], &result[index+1], strlen(&result[index+1]) + 1);
        } else {
            // This shouldn't happen based on problem constraints
            memmove(&result[index], &result[index+1], strlen(&result[index+1]) + 1);
        }
    }
    return result;
}

int main() {
    char s[100001];
    fgets(s, sizeof(s), stdin);
    s[strcspn(s, "\n")] = 0;
    
    char* result = solution(s);
    printf("%s\n", result);
    free(result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n²)

For each of n characters, we may need to scan and rebuild the string

⚠ Quadratic Growth

Space Complexity

O(n)

Creating new strings during manipulation

⚡ Linearithmic Space

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force String Manipulation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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