Minimum steps to reach end of array under constraints

Given an array containing one digit numbers only, assuming we are standing at first index, we need to reach to end of array using minimum number of steps where in one step, we can jump to neighbor indices or can jump to a position with same value.

In other words, if we are at index i, then in one step you can reach to, arr[i-1] or arr[i+1] or arr[K] such that arr[K] = arr[i] (value of arr[K] is same as arr[i])

Examples:



Input : arr[] = {5, 4, 2, 5, 0}
Output : 2
Explanation : Total 2 step required.
We start from 5(0), in first step jump to next 5 
and in second step we move to value 0 (End of arr[]).

Input  : arr[] = [0, 1, 2, 3, 4, 5, 6, 7, 5, 4,
                 3, 6, 0, 1, 2, 3, 4, 5, 7]
Output : 5
Explanation : Total 5 step required.
0(0) -> 0(12) -> 6(11) -> 6(6) -> 7(7) ->
(18)                                                          
(inside parenthesis indices are shown)

This problem can be solved using BFS. We can consider the given array as unweighted graph where every vertex has two edges to next and previous array elements and more edges to array elements with same values. Now for fast processing of third type of edges, we keep 10 vectors which store all indices where digits 0 to 9 are present. In above example, vector corresponding to 0 will store [0, 12], 2 indices where 0 has occurred in given array.
Another Boolean array is used, so that we don’t visit same index more than once. As we are using BFS and BFS proceeds level by level, optimal minimum steps are guaranteed.

C++

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// C++ program to find minimum jumps to reach end
// of array
#include <bits/stdc++.h>
using namespace std;
  
//  Method returns minimum step to reach end of array
int getMinStepToReachEnd(int arr[], int N)
{
    // visit boolean array checks whether current index
    // is previously visited
    bool visit[N];
  
    // distance array stores distance of current
    // index from starting index
    int distance[N];
  
    // digit vector stores indicies where a
    // particular number resides
    vector<int> digit[10];
  
    //  In starting all index are unvisited
    memset(visit, false, sizeof(visit));
  
    //  storing indices of each number in digit vector
    for (int i = 1; i < N; i++)
        digit[arr[i]].push_back(i);
  
    //  for starting index distance will be zero
    distance[0] = 0;
    visit[0] = true;
  
    // Creating a queue and inserting index 0.
    queue<int> q;
    q.push(0);
  
    //  loop untill queue in not empty
    while(!q.empty())
    {
        // Get an item from queue, q.
        int idx = q.front();        q.pop();
  
        //  If we reached to last index break from loop
        if (idx == N-1)
            break;
  
        // Find value of dequeued index
        int d = arr[idx];
  
        // looping for all indices with value as d.
        for (int i = 0; i<digit[d].size(); i++)
        {
            int nextidx = digit[d][i];
            if (!visit[nextidx])
            {
                visit[nextidx] = true;
                q.push(nextidx);
  
                //  update the distance of this nextidx
                distance[nextidx] = distance[idx] + 1;
            }
        }
  
        // clear all indices for digit d, because all
        // of them are processed
        digit[d].clear();
  
        //  checking condition for previous index
        if (idx-1 >= 0 && !visit[idx - 1])
        {
            visit[idx - 1] = true;
            q.push(idx - 1);
            distance[idx - 1] = distance[idx] + 1;
        }
  
        //  checking condition for next index
        if (idx + 1 < N && !visit[idx + 1])
        {
            visit[idx + 1] = true;
            q.push(idx + 1);
            distance[idx + 1] = distance[idx] + 1;
        }
    }
  
    //  N-1th position has the final result
    return distance[N - 1];
}
  
//  driver code to test above methods
int main()
{
    int arr[] = {0, 1, 2, 3, 4, 5, 6, 7, 5,
                 4, 3, 6, 0, 1, 2, 3, 4, 5, 7};
    int N = sizeof(arr) / sizeof(int);
    cout << getMinStepToReachEnd(arr, N);
    return 0;
}

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Python3

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# Python 3 program to find minimum jumps to reach end# of array
  
# Method returns minimum step to reach end of array
def getMinStepToReachEnd(arr,N):
      
    # visit boolean array checks whether current index
    # is previously visited
    visit = [False for i in range(N)]
  
    # distance array stores distance of current
    # index from starting index
    distance = [0 for i in range(N)]
  
    # digit vector stores indicies where a
    # particular number resides
    digit = [[0 for i in range(N)] for j in range(10)]
  
    # storing indices of each number in digit vector
    for i in range(1,N):
        digit[arr[i]].append(i)
  
    # for starting index distance will be zero
    distance[0] = 0
    visit[0] = True
  
    # Creating a queue and inserting index 0.
    q = []
    q.append(0)
  
    # loop untill queue in not empty
    while(len(q)> 0):
          
        # Get an item from queue, q.
        idx = q[0]
        q.remove(q[0])
  
        # If we reached to last index break from loop
        if (idx == N-1):
            break
  
        # Find value of dequeued index
        d = arr[idx]
  
        # looping for all indices with value as d.
        for i in range(len(digit[d])):
            nextidx = digit[d][i]
            if (visit[nextidx] == False):
                visit[nextidx] = True
                q.append(nextidx)
  
                # update the distance of this nextidx
                distance[nextidx] = distance[idx] + 1
  
        # clear all indices for digit d, because all
        # of them are processed
  
        # checking condition for previous index
        if (idx-1 >= 0 and visit[idx - 1] == False):
            visit[idx - 1] = True
            q.append(idx - 1)
            distance[idx - 1] = distance[idx] + 1
  
        # checking condition for next index
        if (idx + 1 < N and visit[idx + 1] == False):
            visit[idx + 1] = True
            q.append(idx + 1)
            distance[idx + 1] = distance[idx] + 1
  
    # N-1th position has the final result
    return distance[N - 1]
  
# driver code to test above methods
if __name__ == '__main__':
    arr = [0, 1, 2, 3, 4, 5, 6, 7, 5, 4, 3, 6, 0, 1, 2, 3, 4, 5, 7]
    N = len(arr)
    print(getMinStepToReachEnd(arr, N))
      
# This code is contributed by
# Surendra_Gangwar

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Output:

5

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Improved By : SURENDRA_GANGWAR



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