Learn and practice the Two Pointers and Modified Binary Search patterns to solve coding interview problems.

Speedrun the Oracle Coding Interview: Top Problems in Python

This module groups together two of the most common patterns that may be used to solve problems where the input data is linear: Two Pointers and Modified Binary Search. Both the Two Pointers and the Modified Binary Search patterns derive their power from their ability to efficiently reduce the size of the search space, that is, both exemplify the divide and conquer approach to problem-solving. After completing this module, we will have the knowledge of how to use this set of related patterns to solve a diverse range of problems.

Two Pointers & Co.

# Statement

Given an array, `colors`, which contains a combination of the following three elements: 

- $0$ (representing red) 

- $1$ (representing white)

- $2$ (representing blue)

Sort the array in place so that the elements of the same color are adjacent, with the colors in the order of red, white, and blue. To improve your problem-solving skills, do not utilize the built-in sort function.

**Constraints:**

- $1 \leq$ `colors.length` $\leq$ 300
- `colors[i]` can only contain $0$s, $1$s, or $2$s.

# Examples

# Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

What is the output if the following array is given as input?

colors = [2, 2, 1, 1, 0]

What is the output if the following array is given as input?
colors = [2, 2, 1, 1, 0]

What is the output if the following array is given as input?

[2, 2, 0, 1, 2, 2, 0, 2]

What is the output if the following array is given as input?
[2, 2, 0, 1, 2, 2, 0, 2]

What is the output if the following array is given as input?

[0, 0, 1, 0, 1, 1, 1, 2, 1, 2]

What is the output if the following array is given as input?
[0, 0, 1, 0, 1, 1, 1, 2, 1, 2]

Sort Colors

Sort Colors

# Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

# Try it yourself

Implement your solution in the following coding playground:

>**Note:** Leave the `return` statement intact while adding your code.

Help me debug only the function Sort Colors . . I don't need the corrected code.I don't want explanations or code review. Just provide me the exact and to-the-point error that helps me run the code successfully on the test cases provided.

Please help me understand why this code failed for the last failed test case. Don't give me the corrected code. . If yes, then Just provide me the exact and to-the-point error that helps me run the code successfully on the test cases provided.

Your current task is to review only the sort_colors function and the other function(s) invoked in it/them. Do not review extra/helper files or functions not invoked in the solution function sort_colors. Do not review the code of data structures objects instantiated like a binary tree, linked list, trie, union find, etc., in the solution functions. Just review the solution/algorithm function of the problem. Also, Answer the following questions: Is the Two Pointers pattern used? Take your time and review the code.

Please provide a brief time complexity analysis of the function sort_colors. While calculating time complexity, include the time complexity of built-in Python functions invoked by the code. The Answer should be to the point and in a few lines.

Please provide a brief Space complexity analysis of the function sort_colors. While calculating space complexity, include the space complexity of built-in Python functions invoked by the code. Do not include the space used for the input parameters and the space used for just returning an output(not used as a scratch space). The Answer should be to the point and in a few lines

def evaluate_results(self) -> object:
 """
 self.__inputs is the list of inputs that are passed as input to function
 self.__actual_output is a list of outputs that is produced by the learner's code
 return : self.__edu__result object
 """
 def sort_colors(colors):
 red, white, blue = 0, 0, len(colors) - 1
 
 while white <= blue:
 if colors[white] == 0:
 if colors[red] != 0:
 colors[red], colors[white] = colors[white], colors[red]
 
 white += 1
 red += 1

 elif colors[white] == 1:
 white += 1

 else:
 if colors[blue] != 2:
 colors[white], colors[blue] = colors[blue], colors[white]

 blue -= 1

 return colors
 



 # multiple outputs will be added at each index respectively for a single test_case
 expected_outputs = [] 
 expected = sort_colors(self.__inputs[0])
 actual = self.__actual_outputs[0]
 # store the 1st expected output that should be displayed to the user 
 expected_outputs.append(expected) 
 if expected == actual:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

 return self.__edu_result

def sort_colors(colors):
    
  # Replace this placeholder return statement with your code  
  return colors

PrivateTestCases.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1: [0, 1, 0]
    output:
    - 1 : [0, 0, 1]
  - name: "testcase 2"
    inputs:
    - 1 : [1]
    output: 
    - 1 : [1]
  - name: "testcase 3"
    inputs:
    - 1: [2, 2]
    output:
    - 1 : [2, 2]
  - name: "testcase 4"
    inputs:
    - 1: [1, 1, 0, 2]
    output:
    - 1 : [0, 1, 1, 2]
  - name: "testcase 5"
    inputs:
    - 1: [2, 1, 1, 0, 0]
    output:
    - 1 : [0, 0, 1, 1, 2]

def sum(input_1, input_2):
    return input_1+input_2

def validate_inputs(self) -> None:
 """
 self.__inputs is the list of inputs that are passed as input to the function.
 Store the result using the self.store_result(error_msg, status).
 Return: void
 """
 def is_valid_1d_array(arr):
 if not isinstance(arr, list):
 return False # Not a valid 1D array if it's not a list
 
 return all(isinstance(elem, int) and not isinstance(elem, bool) for elem in arr)

 # Inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 if not is_valid_1d_array(self.__inputs[0]):
 self.store_result(("Invalid test case."), ValidationFunction.FAIL)
 elif len(self.__inputs[0]) < 1 or len(self.__inputs[0]) > 300:
 self.store_result("Length of array should be between 1 and 300", ValidationFunction.FAIL)
 elif any(x not in [0, 1, 2] for x in self.__inputs[0]):
 self.store_result("Array can only contain 0s, 1s and 2s.", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Two Pointers

Modified Binary Search

Conclusion

Sort Colors

Statement

Examples

Understand the problem

Figure it out!

Try it yourself