Check duplicate numbers return 1 if true, return 0 if false in C++ (Array Assignment) - Full Explaination

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✅ Full Teacher’s Code First:

#include <iostream>
#include <vector>
#include <algorithm> // for sort

using namespace std;

// Function to check for duplicates
bool containsDuplicate(vector<int> nums) {
    sort(nums.begin(), nums.end()); // Step 1: Sort the array

    for (int i = 1; i < nums.size(); i++) { // Step 2: Check adjacent elements
        if (nums[i - 1] == nums[i]) {
            return true; // Duplicate found
        }
    }

    return false; // No duplicates found
}

int main() {
    vector<int> numbers = {1, 1, 2, 3, 4, 5}; // Sample input

    if (containsDuplicate(numbers)) {
        cout << "Duplicate found" << endl;
    } else {
        cout << "No duplicates" << endl;
    }

    return 0;
}

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🧠 Step-by-step Breakdown:

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🟩 #include and setup:

#include <iostream>
#include <vector>
#include <algorithm>

• iostream: For cin, cout.

• vector: A dynamic array that grows/shrinks.

• algorithm: Contains sort() function.

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🟦 Function Header:

bool containsDuplicate(vector<int> nums)

• This function takes a copy of a vector of integers.

• It returns a bool — true if a duplicate is found, false otherwise.

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🟨 Step 1: Sort the array

sort(nums.begin(), nums.end());

• Sorting makes it easy to compare each number with its neighbor.

• After sorting: {1, 1, 2, 3, 4, 5}

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🟧 Step 2: Check each pair

for (int i = 1; i < nums.size(); i++) {
    if (nums[i - 1] == nums[i]) {
        return true;
    }
}

• Start from index 1 (second element)

• Compare nums[i - 1] and nums[i]

• If they are equal, it means a duplicate exists.

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🟥 Step 3: No duplicates? Return false

return false;

• If loop finishes and no duplicate is found, return false.

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🟦 main() Function:

int main() {
    vector<int> numbers = {1, 1, 2, 3, 4, 5};

    if (containsDuplicate(numbers)) {
        cout << "Duplicate found" << endl;
    } else {
        cout << "No duplicates" << endl;
    }

    return 0;
}

• Create a vector numbers

• Call the function containsDuplicate(numbers)

• Print message based on the result

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🔁 Example Runs:

Input	Output
{1, 1, 2, 3}	Duplicate found
{1, 2, 3, 4}	No duplicates
{9, 8, 9, 7}	Duplicate found

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🔍 Summary of Logic:

Step	What it does
1	Sort the array
2	Loop through it
3	Compare each number to its previous one
4	If equal → duplicate found
5	Else → return false

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