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Minimize flips required to make all shortest paths from top-left to bottom-right of a binary matrix equal to S
• Difficulty Level : Easy
• Last Updated : 11 Sep, 2020

Given a binary matrix mat[][] having dimensions N * M and a binary string S of length N + M – 1 , the task is to find the minimum number of flips required to make all shortest paths from the top-left cell to the bottom-right cell equal to the given string S.

Examples:

Input: mat[][] = [[1, 0, 1, 1], [1, 1, 1, 0]], S = “10010”
Output:
Explanation:
Step 1: [[1, 0, 1, 1], [1, 1, 1, 0]] -> [[1, 0, 1, 1], [0, 1, 1, 0]]
Step 2: [[1, 0, 1, 1], [0, 1, 1, 0]] -> [[1, 0, 0, 1], [0, 1, 1, 0]]
Step 3: [[1, 0, 0, 1], [0, 1, 1, 0]] -> [[1, 0, 0, 1], [0, 0, 1, 0]]
Once the above steps are performed, every shortest path from the top-left to bottom-right cell are equal to S.
Therefore, the required count is 3.

Input: mat[][] = [[1, 0, 0, 1, 0]], S = “01101”
Output: 5

Naive Approach: The simplest approach is to generate all possible flips in each cell of the given matrix recursively and check which combination of the minimum flips generates the matrix satisfying the required condition.

Time Complexity: O(2N * M)
Auxiliary Space: O(N * M)

Efficient Approach: To optimize the above approach, the idea is to traverse the matrix and observe that if (i, j) is the current index of the given matrix then, this position will be in the shortest path string at index (i + j) where, i ∈ [0, N-1] and j ∈ [0, M-1]

Follow the steps below to solve the problem:

1. Initialize the counter as 0.
2. Traverse through each position of the matrix arr[][].
3. If the current position in the given matrix is (i, j) then, this position is in the shortest path string at (i + j)th index.
4. At each position, compare arr[i][j] and S[i + j]. If found to be equal, continue to the next position. Otherwise, increase the count by 1.
5. Once the above steps are performed for the entire matrix, print the value of count as the minimum flips required.

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach`` ` `#include ``using` `namespace` `std;`` ` `// Function to count the minimum``// number of flips required``int` `minFlips(vector >& mat,``             ``string s)``{``    ``// Dimensions of matrix``    ``int` `N = mat.size();``    ``int` `M = mat.size();`` ` `    ``// Stores the count the flips``    ``int` `count = 0;`` ` `    ``for` `(``int` `i = 0; i < N; i++) {`` ` `        ``for` `(``int` `j = 0; j < M; j++) {`` ` `            ``// Check if element is same``            ``// or not``            ``if` `(mat[i][j]``                ``!= s[i + j] - ``'0'``) {``                ``count++;``            ``}``        ``}``    ``}`` ` `    ``// Return the final count``    ``return` `count;``}`` ` `// Driver Code``int` `main()``{``    ``// Given Matrix``    ``vector > mat``        ``= { { 1, 0, 1 },``            ``{ 0, 1, 1 },``            ``{ 0, 0, 0 } };`` ` `    ``// Given path as a string``    ``string s = ``"10001"``;`` ` `    ``// Function Call``    ``cout << minFlips(mat, s);`` ` `    ``return` `0;``}`

## Java

 `// Java program for the above approach``class` `GFG {`` ` `    ``// Function to count the minimum``    ``// number of flips required``    ``static` `int` `minFlips(``int` `mat[][], ``                        ``String s)``    ``{``        ``// Dimensions of matrix``        ``int` `N = mat.length;``        ``int` `M = mat[``0``].length;`` ` `        ``// Stores the count the flips``        ``int` `count = ``0``;`` ` `        ``for` `(``int` `i = ``0``; i < N; i++) ``        ``{``            ``for` `(``int` `j = ``0``; j < M; j++) ``            ``{``                ``// Check if element is same``                ``// or not``                ``if` `(mat[i][j] != ``                    ``s.charAt(i + j) - ``'0'``) ``                ``{``                    ``count++;``                ``}``            ``}``        ``}`` ` `        ``// Return the final count``        ``return` `count;``    ``}`` ` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``// Given Matrix``        ``int` `mat[][] = {{``1``, ``0``, ``1``}, ``                       ``{``0``, ``1``, ``1``}, {``0``, ``0``, ``0``}};`` ` `        ``// Given path as a string``        ``String s = ``"10001"``;`` ` `        ``// Function Call``        ``System.out.print(minFlips(mat, s));``    ``}``}`` ` `// This code is contributed by Chitranayal`

## Python3

 `# Python3 program for the above approach `` ` `# Function to count the minimum``# number of flips required ``def` `minFlips(mat, s):`` ` `    ``# Dimensions of matrix``    ``N ``=` `len``(mat)``    ``M ``=` `len``(mat[``0``])`` ` `    ``# Stores the count the flips``    ``count ``=` `0`` ` `    ``for` `i ``in` `range``(N):``        ``for` `j ``in` `range``(M):`` ` `            ``# Check if element is same``            ``# or not``            ``if``(mat[i][j] !``=` `ord``(s[i ``+` `j]) ``-``                            ``ord``(``'0'``)):``                ``count ``+``=` `1`` ` `    ``# Return the final count``    ``return` `count`` ` `# Driver Code`` ` `# Given Matrix``mat ``=` `[ [ ``1``, ``0``, ``1` `],``        ``[ ``0``, ``1``, ``1` `],``        ``[ ``0``, ``0``, ``0` `] ]`` ` `# Given path as a string``s ``=` `"10001"`` ` `# Function call``print``(minFlips(mat, s))`` ` `# This code is contributed by Shivam Singh`

## C#

 `// C# program for the above approach``using` `System;`` ` `class` `GFG{`` ` `// Function to count the minimum``// number of flips required``static` `int` `minFlips(``int` `[,]mat, ``                    ``String s)``{``     ` `    ``// Dimensions of matrix``    ``int` `N = mat.GetLength(0);``    ``int` `M = mat.GetLength(1);`` ` `    ``// Stores the count the flips``    ``int` `count = 0;`` ` `    ``for``(``int` `i = 0; i < N; i++) ``    ``{``        ``for``(``int` `j = 0; j < M; j++) ``        ``{``             ` `            ``// Check if element is same``            ``// or not``            ``if` `(mat[i, j] != ``                  ``s[i + j] - ``'0'``) ``            ``{``                ``count++;``            ``}``        ``}``    ``}`` ` `    ``// Return the readonly count``    ``return` `count;``}`` ` `// Driver Code``public` `static` `void` `Main(String[] args)``{``     ` `    ``// Given Matrix``    ``int` `[,]mat = { { 1, 0, 1 }, ``                   ``{ 0, 1, 1 },``                   ``{ 0, 0, 0 } };`` ` `    ``// Given path as a string``    ``String s = ``"10001"``;`` ` `    ``// Function call``    ``Console.Write(minFlips(mat, s));``}``}`` ` `// This code is contributed by Amit Katiyar`
Output:
```4
```

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

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