Contains Duplicate

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Ever had that feeling of "Haven't I seen you before?" In Contains Duplicate, your mission is to tell us if any value in an array shows up at least twice. If there's even a single pair of twins hidden in the sequence, return true. If every single number is unique and flying solo, return false.

The "secret sauce" here is a Hash Set. Instead of using slow nested loops, you use a set as a smart "seen" list. As you walk through the array, you ask the set: "Is this number already checked in?" If the answer is yes, you found a duplicate! This approach is lightning fast (O(n) time) because set lookups are practically instant. It’s the most elegant and professional way to solve lookup problems in any application. Whether you're scanning log files or IDs, this pattern is a must-master skill!

Keep the algorithm focused on one clear invariant and update path so correctness is easy to verify from left to right. This reduces accidental branching errors and helps ensure the final output stays consistent with the problem contract across random and adversarial test shapes.

Examples

Example 1

Input

nums = [1,2,3,1]

Output

true

Explanation

The value 1 appears more than once.

Example 2

Input

nums = [1,2,3,4]

Output

false

Explanation

All values are distinct.

Example 3

Input

nums = [1,1,1,3,3,4,3,2,4,2]

Output

true

Explanation

Several values repeat, so the answer is true.

Algorithm Flow

Recommendation Algorithm Flow for Contains Duplicate - Budibadu

Best Answers

java

import java.util.*;
class Solution {
    public boolean contains_duplicate(int[] nums) {
        Set<Integer> seen = new HashSet<>();
        for (int num : nums) {
            if (seen.contains(num)) return true;
            seen.add(num);
        }
        return false;
    }
}

javascript

function contains_duplicate(nums) {
    const seen = new Set();
    for (const num of nums) {
        if (seen.has(num)) return true;
        seen.add(num);
    }
    return false;
}

php

<?php
function contains_duplicate($nums) {
    $seen = [];
    foreach ($nums as $num) {
        if (isset($seen[$num])) return true;
        $seen[$num] = true;
    }
    return false;
}
?>

python

from typing import List

def contains_duplicate(nums: List[int]) -> bool:
    seen = set()
    for num in nums:
        if num in seen:
            return True
        seen.add(num)
    return False

rust

use std::collections::HashSet;
fn contains_duplicate(nums: Vec<i32>) -> bool {
    let mut seen = HashSet::new();
    for num in nums {
        if seen.contains(&num) {
            return true;
        }
        seen.insert(num);
    }
    false
}

typescript

function contains_duplicate(nums: number[]): boolean {
    var seen: { [key: string]: boolean } = {};
    for (var i = 0; i < nums.length; i++) {
        var key = String(nums[i]);
        if (seen[key]) return true;
        seen[key] = true;
    }
    return false;
}

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Contains Duplicate

Examples

Algorithm Flow

Best Answers

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