# 20.05.2026 [2657. Find the Prefix Common Array of Two Arrays]

Duplicates in prefixes

20.05.2026

2657. Find the Prefix Common Array of Two Arrays medium

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Problem TLDR

Duplicates in prefixes

Intuition

Only 50 elements, brute-force works.
We can use HashSets or bitmasks

Approach

* shortest version in Kotlin is O(n^2)
* in Rust we can use zip+scan, or a simple map

Complexity

• Time complexity: O(n^2|n)

• Space complexity: O(n)

Code

    fun findThePrefixCommonArray(a: IntArray, b: IntArray) = 
    (1..a.size).map {a.take(it).intersect(b.take(it)).size}

    pub fn find_the_prefix_common_array(a: Vec<i32>, b: Vec<i32>) -> Vec<i32> {
        let (mut c, mut d) = (0, 0i64);
        (0..a.len()).map(|i|{c|=1<<a[i];d|=1<<b[i];(c&d).count_ones() as _}).collect()
    }