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# Find element position in given monotonic sequence

• Difficulty Level : Medium
• Last Updated : 06 Jun, 2021

Given an integer k and a monotonic increasing sequence:
f(n) = an + bn [log2(n)] + cn^3 where (a = 1, 2, 3, …), (b = 1, 2, 3, …), (c = 0, 1, 2, 3, …)
Here, [log2(n)] means, taking the log to the base 2 and round the value down Thus,
if n = 1, the value is 0.
if n = 2-3, the value is 1.
if n = 4-7, the value is 2.
if n = 8-15, the value is 3.
The task is to find the value n such that f(n) = k, if k doesn’t belong to the sequence then print 0

Note: Values are in such a way that they can be expressed in 64 bits and the three integers a, b and c do not exceed 100.

Examples:

Input: a = 2, b = 1, c = 1, k = 12168587437017
Output: 23001
f(23001) = 12168587437017

Input: a = 7, b = 3, c = 0, k = 119753085330
Output: 1234567890

Naive Approach: Given values of a, b, c, find values of f(n) for every value of n and compare it.

Efficient Approach: Use Binary Search, choose n = (min + max) / 2 where min and max are the minimum and maximum values possible for n then,

• If f(n) < k then increment n.
• If f(n) > k then decrement n.
• If f(n) = k then n is the required answer.
• Repeat the above steps until the required value is found or it is not possible in the sequence.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``#include ``#define SMALL_N 1000000``#define LARGE_N  1000000000000000``using` `namespace` `std;` `// Function to return the value of f(n) for given values of a, b, c, n``long` `long` `func(``long` `long` `a, ``long` `long` `b, ``long` `long` `c, ``long` `long` `n)``{``    ``long` `long` `res = a * n;``    ``long` `long` `logVlaue = ``floor``(log2(n));``    ``res += b * n * logVlaue;``    ``res += c * (n * n * n);``    ``return` `res;``}` `long` `long` `getPositionInSeries(``long` `long` `a, ``long` `long` `b,``                             ``long` `long` `c, ``long` `long` `k)``{``    ``long` `long` `start = 1, end = SMALL_N;` `    ``// if c is 0, then value of n can be in order of 10^15.``    ``// if c!=0, then n^3 value has to be in order of 10^18``    ``// so maximum value of n can be 10^6.``    ``if` `(c == 0) {``        ``end = LARGE_N;``    ``}``    ``long` `long` `ans = 0;` `    ``// for efficient searching, use binary search.``    ``while` `(start <= end) {``        ``long` `long` `mid = (start + end) / 2;``        ``long` `long` `val = func(a, b, c, mid);``        ``if` `(val == k) {``            ``ans = mid;``            ``break``;``        ``}``        ``else` `if` `(val > k) {``            ``end = mid - 1;``        ``}``        ``else` `{``            ``start = mid + 1;``        ``}``    ``}``    ``return` `ans;``}` `// Driver code``int` `main()``{``    ``long` `long` `a = 2, b = 1, c = 1;``    ``long` `long` `k = 12168587437017;` `    ``cout << getPositionInSeries(a, b, c, k);` `    ``return` `0;``}`

## Python3

 `# Python 3 implementation of the approach``from` `math ``import` `log2, floor``SMALL_N ``=` `1000000``LARGE_N ``=` `1000000000000000` `# Function to return the value of f(n)``# for given values of a, b, c, n``def` `func(a, b, c, n) :``    ` `    ``res ``=` `a ``*` `n``    ``logVlaue ``=` `floor(log2(n))``    ``res ``+``=` `b ``*` `n ``*` `logVlaue``    ``res ``+``=` `c ``*` `(n ``*` `n ``*` `n)``    ``return` `res` `def` `getPositionInSeries(a, b, c, k) :``    ` `    ``start ``=` `1``    ``end ``=` `SMALL_N` `    ``# if c is 0, then value of n``    ``# can be in order of 10^15.``    ``# if c!=0, then n^3 value has``    ``# to be in order of 10^18``    ``# so maximum value of n can be 10^6.``    ``if` `(c ``=``=` `0``) :``        ``end ``=` `LARGE_N``    ` `    ``ans ``=` `0` `    ``# for efficient searching,``    ``# use binary search.``    ``while` `(start <``=` `end) :``        ` `        ``mid ``=` `(start ``+` `end) ``/``/` `2``        ``val ``=` `func(a, b, c, mid)``        ``if` `(val ``=``=` `k) :``            ``ans ``=` `mid``            ``break``    ` `        ``elif` `(val > k) :``            ``end ``=` `mid ``-` `1` `        ``else` `:``            ``start ``=` `mid ``+` `1``        ` `    ``return` `ans;` `# Driver code``if` `__name__ ``=``=` `"__main__"` `:``    ` `    ``a ``=` `2``    ``b ``=` `1``    ``c ``=` `1``    ``k ``=` `12168587437017` `    ``print``(getPositionInSeries(a, b, c, k))` `# This code is contributed by Ryuga`

## PHP

 ` ``\$k``)``            ``\$end` `= ``\$mid` `- 1;` `        ``else``            ``\$start` `= ``\$mid` `+ 1;``    ``}``    ``return` `\$ans``;``}` `// Driver code``\$a` `= 2;``\$b` `= 1;``\$c` `= 1;``\$k` `= 12168587437017;` `print``(getPositionInSeries(``\$a``, ``\$b``, ``\$c``, ``\$k``));` `// This code is contributed by mits``?>`

## Javascript

 ``
Output:
`23001`

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