Program to check we can reach leftmost or rightmost position or not in Python

Suppose we have a string containing letters of three types: R, B, and dot(.). Here R stands for our current position, B stands for a blocked position, and dot(.) stands for an empty position. In one step, we can move to any adjacent position to our current position, as long as it is valid (empty). We have to check whether we can reach either the leftmost position or the rightmost position.

So, if the input is like s = "...........R.....BBBB.....", then the output will be True, as R can reach the leftmost position because there is no block between R and the left edge.

Algorithm

To solve this, we will follow these steps ?

• Find the index position of 'R' in the string
• Check if there are no 'B' characters between the start and R position (can reach left)
• Check if there are no 'B' characters between R position and the end (can reach right)
• Return True if either condition is satisfied

Example

Let us see the following implementation to get better understanding ?

class Solution:
    def solve(self, s):
        r_pos = s.find('R')
        # Check if we can reach leftmost OR rightmost position
        return 'B' not in s[:r_pos] or 'B' not in s[r_pos:]

# Test the solution
ob = Solution()
s = "...........R.....BBBB....."
print(ob.solve(s))

True

How It Works

The solution works by checking two conditions:

• Left reachability: 'B' not in s[:r_pos] checks if there are no blocked positions between the start and R
• Right reachability: 'B' not in s[r_pos:] checks if there are no blocked positions from R to the end

If either condition is true, we can reach at least one edge, so the function returns True.

Additional Examples

class Solution:
    def solve(self, s):
        r_pos = s.find('R')
        return 'B' not in s[:r_pos] or 'B' not in s[r_pos:]

ob = Solution()

# Test case 1: Can reach both sides
print("Test 1:", ob.solve("....R...."))

# Test case 2: Can only reach right side
print("Test 2:", ob.solve("BB..R...."))

# Test case 3: Cannot reach either side
print("Test 3:", ob.solve("BB.R.BB"))

# Test case 4: Can only reach left side
print("Test 4:", ob.solve("....R..BB"))

Test 1: True
Test 2: True
Test 3: False
Test 4: True

Conclusion

This solution efficiently checks reachability in O(n) time by using string slicing and the in operator. The key insight is that we only need to reach either the leftmost or rightmost position, not both.