# Dynamic Programming Algorithms

## Dynamic Programming Algorithms​

Dynamic programming is a method used in computer science and mathematics to solve complex problems by breaking them down into simpler subproblems. It is particularly useful for optimization problems where the solution can be constructed efficiently from solutions to overlapping subproblems.

### Key Concepts of Dynamic Programming:​

Optimal Substructure: This property means that the solution to a given problem can be constructed from the solutions of its subproblems. If a problem has an optimal substructure, dynamic programming can be used to solve it efficiently.

Overlapping Subproblems: Dynamic programming is applicable when the problem can be broken down into subproblems which are reused several times. Unlike divide-and-conquer methods which solve subproblems independently, dynamic programming solves each subproblem once and stores the result to avoid redundant computations.

#### Approach:​

**Top-Down Approach (Memoization): **In this approach, the problem is solved by breaking it down recursively, but intermediate results are stored (memoized) to avoid redundant calculations. This is typically implemented using recursion along with a data structure (like a dictionary or array) to store the results of subproblems.

**Bottom-Up Approach (Tabulation): **This approach involves solving all possible subproblems, starting with the simplest ones and gradually combining them to solve larger subproblems. This is usually implemented iteratively using a table to store the solutions to subproblems.

#### Examples:​

Fibonacci Sequence: Calculating Fibonacci numbers using dynamic programming involves storing the results of previous calculations to avoid redundant computations.

Knapsack Problem: Determining the maximum value that can be carried in a knapsack of fixed capacity, given weights and values of items, can be solved efficiently using dynamic programming.

Longest Common Subsequence: Finding the longest subsequence common to two sequences can be optimized using dynamic programming by storing the lengths of common subsequences of various prefixes.

#### Benefits of Dynamic Programming:​

Efficiency: By storing the results of overlapping subproblems, dynamic programming reduces the time complexity of solving problems, often converting exponential time algorithms into polynomial time algorithms.

Simplicity: Dynamic programming simplifies the process of solving complex problems by breaking them down into simpler, more manageable subproblems.

## Recursive Functions​

A recursive function is a function that calls itself in order to solve a problem. The process of a function calling itself is known as recursion. Recursive functions typically solve a problem by breaking it down into smaller, more manageable subproblems of the same type, until they reach a base case, which is a condition that stops the recursion. Key Components:

Base Case: The condition under which the function stops calling itself. It prevents infinite recursion and provides a simple, direct solution to the smallest instance of the problem.

Recursive Case: The part of the function where it calls itself with a smaller or simpler input, moving towards the base case.

### Sample Recursive Function:​

Here is a simple JavaScript implementation of a recursive function to calculate the factorial of a number:

``function factorial(n) {  // Base case: if n is 0, return 1  if (n === 0) {    return 1;  }  // Recursive case: n * factorial of (n - 1)  return n * factorial(n - 1);}// Example usageconsole.log(factorial(5)); // Output: 120``

Explanation:

Base Case: If `n` is 0, the function returns 1, as the factorial of 0 is defined to be 1.

Recursive Case: For any `n>0`, the function calls itself with `n−1` and multiplies the result by `n`. This process repeats until the base case is reached.

For factorial(5), the function calls proceed as follows:

factorial(5) calls factorial(4) factorial(4) calls factorial(3) factorial(3) calls factorial(2) factorial(2) calls factorial(1) factorial(1) calls factorial(0) factorial(0) returns 1

Then, the results are multiplied back up:

factorial(1) returns 1 _ 1 = 1 factorial(2) returns 2 _ 1 = 2 factorial(3) returns 3 _ 2 = 6 factorial(4) returns 4 _ 6 = 24 factorial(5) returns 5 * 24 = 120

Thus, factorial(5) returns 120.

## Recursive Backtracking​

Backtracking is a technique used to solve problems by exploring all possible solutions and "backtracking" when a solution is found to be invalid. Recursive backtracking involves using recursion to explore the solution space, trying out different choices at each step and backtracking when a choice leads to a dead end.

Key Concepts:

• Choice: Decide which option to take at each step.

• Constraint: Determine whether the current choice is valid.

• Goal: Determine whether the current solution meets the problem's requirements.

Example Problem:

A classic problem that can be solved using recursive backtracking is the N-Queens problem. The objective is to place N queens on an N×N chessboard such that no two queens threaten each other.

``function solveNQueens(n) {  const result = [];  const board = Array.from({length: n}, () => Array(n).fill('.'));  function isSafe(board, row, col) {    for (let i = 0; i < row; i++) {      if (board[i][col] === 'Q') return false;    }    for (let i = row, j = col; i >= 0 && j >= 0; i--, j--) {      if (board[i][j] === 'Q') return false;    }    for (let i = row, j = col; i >= 0 && j < n; i--, j++) {      if (board[i][j] === 'Q') return false;    }    return true;  }  function backtrack(row) {    if (row === n) {      const copy = board.map((row) => row.join(''));      result.push(copy);      return;    }    for (let col = 0; col < n; col++) {      if (isSafe(board, row, col)) {        board[row][col] = 'Q';        backtrack(row + 1);        board[row][col] = '.';      }    }  }  backtrack(0);  return result;}// Example usageconst solutions = solveNQueens(4);console.log(solutions);``

Explanation:

Board Initialization: The board is initialized as an `N×N` grid filled with '.', representing empty spaces.

Safety Check: The isSafe function checks if a queen can be placed at board[row][col] without being threatened by another queen. It checks:

• The column above the current position.
• The upper left diagonal.
• The upper right diagonal.

Backtracking Function: The backtrack function places queens row by row:

• If row equals n, a valid configuration is found, and it's added to the result.
• For each column in the current row, it checks if placing a queen there is safe.
• If it's safe, the queen is placed, and backtrack is called for the next row.
• After exploring all possibilities for the next row, the queen is removed (backtracking) to try the next column in the current row.

Result: The function returns all valid configurations of the N-Queens problem.

For `n = 4`, the solution will output two valid configurations where no two queens threaten each other:

### Objective​

The goal of Kadane's Algorithm is to find the maximum sum of a contiguous subarray within a given one-dimensional numeric array. This algorithm is used to solve the maximum subarray problem.

### Concept​

Kadane's Algorithm uses a dynamic programming approach to solve the problem in linear time. It maintains two variables:

`current_max`: the maximum sum of the subarray ending at the current position.

`global_max`: the maximum sum found so far across all subarrays.

### Steps​

1. Initialize current_max and global_max to the first element of the array.
2. Iterate through the array from the second element to the end.
3. For each element, update current_max as the maximum of the current element itself and the sum of current_max and the current element. This step decides whether to include the current element in the existing subarray or start a new subarray.
4. Update global_max as the maximum of global_max and current_max.
5. At the end of the iteration, global_max will hold the maximum sum of the contiguous subarray.

### Pseudocode​

``    Initialize:        max_so_far = INT_MIN        max_ending_here = 0    Loop for each element of the array      (a) max_ending_here = max_ending_here + a[i]      (b) if(max_so_far < max_ending_here)                max_so_far = max_ending_here      (c) if(max_ending_here < 0)                max_ending_here = 0    return max_so_far``
``function kadaneAlgorithm(arr) {  // Initialize the current maximum and global maximum to the first element  let currentMax = arr[0];  let globalMax = arr[0];  // Iterate through the array starting from the second element  for (let i = 1; i < arr.length; i++) {    // Update current maximum to be the maximum of the current element or the sum of currentMax and the current element    currentMax = Math.max(arr[i], currentMax + arr[i]);    // Update global maximum if the current maximum is greater    if (currentMax > globalMax) {      globalMax = currentMax;    }  }  // Return the global maximum, which is the maximum sum of a contiguous subarray  return globalMax;}// Example usage:const array = [-2, 1, -3, 4, -1, 2, 1, -5, 4];console.log(kadaneAlgorithm(array)); // Output: 6``

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