LeetCode Entry

3129. Find All Possible Stable Binary Arrays I

09.03.2026 medium 2026 kotlin

Count 01-arrays with z zeros, o ones, at most l repeat

3129. Find All Possible Stable Binary Arrays I medium blog post substack youtube

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Problem TLDR

Count 01-arrays with z zeros, o ones, at most l repeat #medium

Intuition

Didn’t solve.

    // length = z+o = 400
    // consequent repeats at most l
    // this can be DFS + memo n^3
    // my solution is n^4 TLE
    // the symmetry trick didn't help
    // lets look hints
    // MLE

The working intuition: build arrays by alterating blocks.

Approach

• inside DFS: try at most min(l, current) to take, flip the arguments

Complexity

• Time complexity: \(O(zol)\)

• Space complexity: \(O(zo)\)

Code

// 218ms
    fun numberOfStableArrays(z: Int, o: Int, l: Int): Int {
        val dp = HashMap<Int, Int>()
        fun d(z: Int, o: Int): Int =
        if (z == 0) 0 else if (o == 0) {if (z <= l) 1 else 0}
        else dp.getOrPut(z*400+o) {
            (1..min(z,l)).fold(0){ r, nz -> (r+d(o,z-nz))%1000000007}
        }
        return (d(z, o) + d(o, z))%1000000007
    }