Federico Mengozzi

Yet another subarray quiz

Maximum non adjacent subsequence

Find the non adjacent subsequence with maximum sum

Given an arrays of $A$ size $N$, find a sequence of non adjacent value with the larger sum, the sequence should be something like $A_i, A_{i+k}, A_{i+k’}, \cdots, A_{i+k^n}, \forall k > 1$

It’s easy to solve this problem with dynamic programming

int dp(std::vector<int> &v, int index) {
    if(index >= v.size())
        return 0;

    if(memo[index] != -1)
        return memo[index];

    // including the current value "v[index]" require to skip the next at "index + 1"
    int incl = v[index] + dp(v, index + 2);
    // excluding the current value allows us to consider the next value at "index + 1"
    int excl = dp(v, index + 1);

    memo[index] = std::max(incl, excl);

    return memo[index];
}

Using a similar intuition it’s possible to come up with a linear solution

int linear(std::vector<int> &v) {
    int incl = v[0], excl = 0, tmp;

    for(int i=1; i<N; ++i){
        tmp = incl;
        incl = max(incl, excl + v[i]);
        excl = tmp;
    }
    return std::max(incl, excl);
}

By analyzing all possible cases we see that

  • if incl > excl + v[i] we don’t care for the current value, hence we can decide both to consider the next element or to skip it. So incl is going to be equal to excl
  • if incl < excl + v[i] then incl has the current value, while excl is set to the previous value of incl

Basically at every iteration incl and excl are swapped (except for the case when using a value doesn’t led to a better solution)

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