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Maximum count of integers to be chosen from given two stacks having sum at most K

• Difficulty Level : Hard
• Last Updated : 10 Feb, 2022

Given two stacks stack1[] and stack2[] of size N and M respectively and an integer K, The task is to count the maximum number of integers from two stacks having sum less than or equal to K.

Examples:

Input: stack1[ ] = { 60, 90, 120 }
stack2[ ] = { 100, 10, 10, 250 }, K = 130
Output: 3
Explanation: Take 3 numbers from stack1 which are 100 and 10 and 10.
Total sum 100 + 10 + 10 = 120

Input: stack1[ ] = { 60, 90, 120 }
stack2[ ] = { 80, 150, 80, 150 }, K = 740
Output: 7
Explanation: Select all the numbers because the value K is enough.

Approach: This problem cannot be solved using the Greedy approach because in greedy at each step the number having the minimum value will be selected but the first example fails according to this. This problem can be solved using prefix sum and binary search. calculate the prefix sum of both the stacks and now iterate for every possible value of 1st stack and take target which is (K – stack1[i]) and apply binary search on the second stack to take lower bound of stack2[].
Follow the steps mentioned below:

• Take two new stacks which are sumA[] and sumB[].
• Calculate the prefix of both the stacks stack1[] and stack2[].
• Iterate on the first stack.
• Now, take the remValueOfK variable and store (K – stack1[i]).
• If it is less than 0 so continue the loop.
• Else take the lower bound of the second stack.
• If the lower bound is greater than the size of the second stack or the value of the lower bound is greater than the value of remValueOfK just decrement the value of the lower bound variable.
• Store the maximum count of elements selected and return that as the final answer.

Below is the implementation of the above approach.

C++

 `// C++ code to implement the above approach` `#include ` `using` `namespace` `std;`   `// Function to find the` `// maximum number of elements` `int` `maxNumbers(``int` `stack1[], ``int` `N, ``int` `stack2[],` `               ``int` `M, ``int` `K)` `{`   `    ``// Take prefix of both the stack` `    ``vector<``int``> sumA(N + 1, 0);` `    ``vector<``int``> sumB(M + 1, 0);` `    ``for` `(``int` `i = 0; i < N; i++)` `        ``sumA[i + 1] = sumA[i] + stack1[i];`   `    ``for` `(``int` `i = 0; i < M; i++)` `        ``sumB[i + 1] = sumB[i] + stack2[i];`   `    ``// Calculate maxNumbers` `    ``int` `MaxNumbers = 0;` `    ``for` `(``int` `i = 0; i <= N; i++) {`   `        ``// Calculate remaining value of K` `        ``// after selecting numbers` `        ``// from 1st stack` `        ``int` `remValueOfK = K - sumA[i];`   `        ``// If rem value of K is less than 0` `        ``// continue the loop` `        ``if` `(remValueOfK < 0)` `            ``continue``;`   `        ``// Calculate lower bound` `        ``int` `lowerBound` `            ``= lower_bound(sumB.begin(),` `                          ``sumB.end(),` `                          ``remValueOfK)` `              ``- sumB.begin();`   `        ``// If size of lower bound is greater` `        ``// than self stack size or` `        ``// value of lower bound element` `        ``// decrement lowerBound` `        ``if` `(lowerBound > M` `            ``or sumB[lowerBound] > remValueOfK) {` `            ``lowerBound--;` `        ``}`   `        ``// Store max possible numbers` `        ``int` `books = i + lowerBound;` `        ``MaxNumbers = max(MaxNumbers, books);` `    ``}` `    ``return` `MaxNumbers;` `}`   `// Driver code` `int` `main()` `{` `    ``int` `stack1[] = { 60, 90, 120 };` `    ``int` `stack2[] = { 100, 10, 10, 200 };` `    ``int` `K = 130;` `    ``int` `N = 3;` `    ``int` `M = 4;` `    ``int` `ans` `        ``= maxNumbers(stack1, N, stack2, M, K);` `    ``cout << ans;` `    ``return` `0;` `}`

Java

 `// Java program to implement` `// the above approach` `import` `java.util.*;`   `class` `GFG` `{`   `  ``static` `int` `lower_bound(``int` `[]a, ``int` `val) {` `    ``int` `lo = ``0``, hi = a.length - ``1``;` `    ``while` `(lo < hi) {` `      ``int` `mid = (``int``)Math.floor(lo + (``double``)(hi - lo) / ``2``);` `      ``if` `(a[mid] < val)` `        ``lo = mid + ``1``;` `      ``else` `        ``hi = mid;` `    ``}` `    ``return` `lo;` `  ``}`   `  ``// Function to find the` `  ``// maximum number of elements` `  ``static` `int` `maxNumbers(``int` `[]stack1, ``int` `N, ``int` `[]stack2,` `                        ``int` `M, ``int` `K) {`   `    ``// Take prefix of both the stack` `    ``int` `[]sumA = ``new` `int``[N + ``1``];` `    ``for``(``int` `i = ``0``; i < N + ``1``; i++) {` `      ``sumA[i] = ``0``;` `    ``}`   `    ``int` `[]sumB = ``new` `int``[M + ``1``];` `    ``for``(``int` `i = ``0``; i < M + ``1``; i++) {` `      ``sumB[i] = ``0``;` `    ``}`   `    ``for` `(``int` `i = ``0``; i < N; i++)` `      ``sumA[i + ``1``] = sumA[i] + stack1[i];`   `    ``for` `(``int` `i = ``0``; i < M; i++)` `      ``sumB[i + ``1``] = sumB[i] + stack2[i];`   `    ``// Calculate maxNumbers` `    ``int` `MaxNumbers = ``0``;` `    ``for` `(``int` `i = ``0``; i <= N; i++) {`   `      ``// Calculate remaining value of K` `      ``// after selecting numbers` `      ``// from 1st stack` `      ``int` `remValueOfK = K - sumA[i];`   `      ``// If rem value of K is less than 0` `      ``// continue the loop` `      ``if` `(remValueOfK < ``0``)` `        ``continue``;`   `      ``// Calculate lower bound` `      ``int` `lowerBound` `        ``= lower_bound(sumB,` `                      ``remValueOfK);`     `      ``// If size of lower bound is greater` `      ``// than self stack size or` `      ``// value of lower bound element` `      ``// decrement lowerBound` `      ``if` `(lowerBound > M` `          ``|| sumB[lowerBound] > remValueOfK) {` `        ``lowerBound--;` `      ``}`   `      ``// Store max possible numbers` `      ``int` `books = i + lowerBound;` `      ``MaxNumbers = Math.max(MaxNumbers, books);` `    ``}` `    ``return` `MaxNumbers;` `  ``}`   `  ``// Driver Code` `  ``public` `static` `void` `main(String args[])` `  ``{` `    ``int` `[]stack1 = {``60``, ``90``, ``120``};` `    ``int` `[]stack2 = {``100``, ``10``, ``10``, ``200``};` `    ``int` `K = ``130``;` `    ``int` `N = ``3``;` `    ``int` `M = ``4``;` `    ``int` `ans = maxNumbers(stack1, N, stack2, M, K);` `    ``System.out.println(ans);`   `  ``}` `}`   `// This code is contributed by sanjoy_62.`

Python3

 `# Python code for the above approach ` `def` `lower_bound(a, val):` `    ``lo ``=` `0` `    ``hi ``=` `len``(a) ``-` `1``;` `    ``while` `(lo < hi):` `        ``mid ``=` `(lo ``+` `(hi ``-` `lo) ``/``/` `2``);` `        ``if` `(a[mid] < val):` `            ``lo ``=` `mid ``+` `1``;` `        ``else``:` `            ``hi ``=` `mid;` `    ``return` `lo;`   `# Function to find the` `# maximum number of elements` `def` `maxNumbers(stack1, N, stack2, M, K):`   `    ``# Take prefix of both the stack` `    ``sumA ``=` `[``0``] ``*` `(N ``+` `1``)` `    ``sumB ``=` `[``0``] ``*` `(M ``+` `1``)` `    ``for` `i ``in` `range``(N):` `        ``sumA[i ``+` `1``] ``=` `sumA[i] ``+` `stack1[i];`   `    ``for` `i ``in` `range``(M):` `        ``sumB[i ``+` `1``] ``=` `sumB[i] ``+` `stack2[i];`   `    ``# Calculate maxNumbers` `    ``MaxNumbers ``=` `0``;` `    ``for` `i ``in` `range``(N ``+` `1``):`   `        ``# Calculate remaining value of K` `        ``# after selecting numbers` `        ``# from 1st stack` `        ``remValueOfK ``=` `K ``-` `sumA[i];`   `        ``# If rem value of K is less than 0` `        ``# continue the loop` `        ``if` `(remValueOfK < ``0``):` `            ``continue``;`   `        ``# Calculate lower bound` `        ``lowerBound ``=` `lower_bound(sumB, remValueOfK);`     `        ``# If size of lower bound is greater` `        ``# than self stack size or` `        ``# value of lower bound element` `        ``# decrement lowerBound` `        ``if` `(lowerBound > M ``or` `sumB[lowerBound] > remValueOfK):` `            ``lowerBound ``-``=` `1`   `        ``# Store max possible numbers` `        ``books ``=` `i ``+` `lowerBound;` `        ``MaxNumbers ``=` `max``(MaxNumbers, books);` `    `  `    ``return` `MaxNumbers;`   `# Driver code`   `stack1 ``=` `[``60``, ``90``, ``120``];` `stack2 ``=` `[``100``, ``10``, ``10``, ``200``];` `K ``=` `130``;` `N ``=` `3``;` `M ``=` `4``;` `ans ``=` `maxNumbers(stack1, N, stack2, M, K);` `print``(ans)`   `# This code is contributed by gfgking`

C#

 `// C# code for the above approach ` `using` `System;` `class` `GFG` `{` `  ``static` `int` `lower_bound(``int` `[]a, ``int` `val) {` `    ``int` `lo = 0, hi = a.Length - 1;` `    ``while` `(lo < hi) {` `      ``int` `mid = (``int``)Math.Floor(lo + (``double``)(hi - lo) / 2);` `      ``if` `(a[mid] < val)` `        ``lo = mid + 1;` `      ``else` `        ``hi = mid;` `    ``}` `    ``return` `lo;` `  ``}`   `  ``// Function to find the` `  ``// maximum number of elements` `  ``static` `int` `maxNumbers(``int` `[]stack1, ``int` `N, ``int` `[]stack2,` `                        ``int` `M, ``int` `K) {`   `    ``// Take prefix of both the stack` `    ``int` `[]sumA = ``new` `int``[N + 1];` `    ``for``(``int` `i = 0; i < N + 1; i++) {` `      ``sumA[i] = 0;` `    ``}`   `    ``int` `[]sumB = ``new` `int``[M + 1];` `    ``for``(``int` `i = 0; i < M + 1; i++) {` `      ``sumB[i] = 0;` `    ``}`   `    ``for` `(``int` `i = 0; i < N; i++)` `      ``sumA[i + 1] = sumA[i] + stack1[i];`   `    ``for` `(``int` `i = 0; i < M; i++)` `      ``sumB[i + 1] = sumB[i] + stack2[i];`   `    ``// Calculate maxNumbers` `    ``int` `MaxNumbers = 0;` `    ``for` `(``int` `i = 0; i <= N; i++) {`   `      ``// Calculate remaining value of K` `      ``// after selecting numbers` `      ``// from 1st stack` `      ``int` `remValueOfK = K - sumA[i];`   `      ``// If rem value of K is less than 0` `      ``// continue the loop` `      ``if` `(remValueOfK < 0)` `        ``continue``;`   `      ``// Calculate lower bound` `      ``int` `lowerBound` `        ``= lower_bound(sumB,` `                      ``remValueOfK);`     `      ``// If size of lower bound is greater` `      ``// than self stack size or` `      ``// value of lower bound element` `      ``// decrement lowerBound` `      ``if` `(lowerBound > M` `          ``|| sumB[lowerBound] > remValueOfK) {` `        ``lowerBound--;` `      ``}`   `      ``// Store max possible numbers` `      ``int` `books = i + lowerBound;` `      ``MaxNumbers = Math.Max(MaxNumbers, books);` `    ``}` `    ``return` `MaxNumbers;` `  ``}`   `  ``// Driver code` `  ``public` `static` `void` `Main() {`   `    ``int` `[]stack1 = {60, 90, 120};` `    ``int` `[]stack2 = {100, 10, 10, 200};` `    ``int` `K = 130;` `    ``int` `N = 3;` `    ``int` `M = 4;` `    ``int` `ans = maxNumbers(stack1, N, stack2, M, K);` `    ``Console.Write(ans);`   `  ``}` `}`   `// This code is contributed by Samim Hossain Mondal.`

Javascript

 ``

Output

`3`

Time Complexity: O(N * logN)
Auxiliary Space: O(N)

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