Shuffle an Array

Shuffle an Array - Problem

Given an integer array nums, design an algorithm to randomly shuffle the array. All permutations of the array should be equally likely as a result of the shuffling.

Implement the Solution class:

Solution(int[] nums) Initializes the object with the integer array nums.
int[] reset() Resets the array to its original configuration and returns it.
int[] shuffle() Returns a random shuffling of the array.

Input & Output

Example 1 — Basic Shuffle

$ Input: nums = [1,2,3]

› Output: [3,1,2] (or any permutation)

💡 Note: Initialize with [1,2,3], shuffle() returns a random permutation like [3,1,2], reset() returns [1,2,3], shuffle() returns another random arrangement

Example 2 — Single Element

$ Input: nums = [1]

› Output: [1]

💡 Note: Only one element, so shuffle() always returns [1], reset() returns [1]

Example 3 — Duplicate Values

$ Input: nums = [1,1,2]

› Output: [2,1,1] (or any permutation)

💡 Note: Even with duplicates, all permutations should be equally likely: [1,1,2], [1,2,1], [2,1,1]

Constraints

1 ≤ nums.length ≤ 50
-10⁶ ≤ nums[i] ≤ 10⁶
All the elements of nums are unique
At most 10⁴ calls will be made to reset and shuffle

Visualization

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Asked in

G Google 15 f Facebook 12 M Microsoft 10 a Amazon 8

The Fisher-Yates shuffle is the optimal solution for randomly shuffling an array. It works by iterating backwards through the array and swapping each element with a randomly chosen element from the remaining unprocessed portion. This guarantees that every permutation has equal probability of 1/n!. Best approach: Fisher-Yates with Time: O(n), Space: O(1).

Common Approaches

✓ Brute Force - Generate Random Permutation

⏱️ Time: O(n!) Space: O(n!)

Create all possible permutations of the array and randomly select one. This ensures equal probability but is extremely inefficient.

Fisher-Yates Shuffle Algorithm

⏱️ Time: O(n) Space: O(1)

Use the Fisher-Yates shuffle algorithm which iterates through the array and swaps each element with a random element from the remaining unshuffled portion.

Brute Force - Generate Random Permutation — Algorithm Steps

Generate all n! permutations
Pick one randomly using random index
Return the selected permutation

Visualization

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Step-by-Step Walkthrough

Generate All

Create all n! permutations

Pick Random

Randomly select one permutation

Result

Return the chosen arrangement

Code -

solution.c — C

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

typedef struct {
    int* original;
    int* array;
    int size;
} Solution;

// Static arrays to avoid stack overflow - support up to 7! = 5040 permutations
static int allPerms[5040][50];
static int permCount;

Solution* solutionCreate(int* nums, int numsSize) {
    Solution* obj = (Solution*)malloc(sizeof(Solution));
    obj->size = numsSize;
    obj->original = (int*)malloc(numsSize * sizeof(int));
    obj->array = (int*)malloc(numsSize * sizeof(int));
    for (int i = 0; i < numsSize; i++) {
        obj->original[i] = nums[i];
        obj->array[i] = nums[i];
    }
    srand(time(NULL));
    return obj;
}

int* solutionReset(Solution* obj, int* retSize) {
    *retSize = obj->size;
    for (int i = 0; i < obj->size; i++) {
        obj->array[i] = obj->original[i];
    }
    return obj->array;
}

// Generate all permutations (brute-force approach)
void generatePermutations(Solution* obj, int* current, int currentSize, int* used) {
    if (currentSize == obj->size) {
        for (int i = 0; i < obj->size; i++) {
            allPerms[permCount][i] = current[i];
        }
        permCount++;
        return;
    }
    
    for (int i = 0; i < obj->size; i++) {
        if (!used[i]) {
            used[i] = 1;
            current[currentSize] = obj->array[i];
            generatePermutations(obj, current, currentSize + 1, used);
            used[i] = 0;
        }
    }
}

int* solutionShuffle(Solution* obj, int* retSize) {
    *retSize = obj->size;
    
    // Generate all permutations
    permCount = 0;
    int current[50];
    int used[50] = {0};
    generatePermutations(obj, current, 0, used);
    
    // Pick a random permutation
    int randomIndex = rand() % permCount;
    for (int i = 0; i < obj->size; i++) {
        obj->array[i] = allPerms[randomIndex][i];
    }
    return obj->array;
}

int* solution(int* nums, int numsSize, int* returnSize) {
    Solution* obj = solutionCreate(nums, numsSize);
    return solutionShuffle(obj, returnSize);
}

int main() {
    char line[1000];
    if (fgets(line, sizeof(line), stdin) == NULL) return 1;
    
    // Parse JSON array format [1,2,3]
    int nums[100];
    int size = 0;
    char* ptr = strchr(line, '[');
    if (ptr) {
        ptr++; // skip '['
        char* end = strchr(ptr, ']');
        if (end) *end = '\0';
        
        // Use strtol for robust parsing
        char* endptr;
        while (*ptr && size < 100) {
            while (*ptr == ' ' || *ptr == ',') ptr++;
            if (*ptr == '\0') break;
            
            nums[size] = (int)strtol(ptr, &endptr, 10);
            if (ptr == endptr) break; // No valid number found
            ptr = endptr;
            size++;
        }
    }
    
    int returnSize;
    int* result = solution(nums, size, &returnSize);
    
    printf("[");
    for (int i = 0; i < returnSize; i++) {
        printf("%d", result[i]);
        if (i < returnSize - 1) printf(",");
    }
    printf("]\n");
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n!)

Need to generate all n! permutations

✓ Linear Growth

Space Complexity

O(n!)

Store all permutations in memory

✓ Linear Space

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force - Generate Random Permutation — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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