Thomas Ngo

Software Engineer

1 minute read

Problem Statement

problem-2485

Intuition

My initial thought is to iterate through the integers, keeping track of the current sum and comparing it with the sum of the remaining integers on the right.

Approach

I’ll use a simple loop to iterate through the integers from 1 to n. At each step, I’ll update the current sum and check if the remaining integers on the right have the same sum. If I find such a pivot integer, I’ll return it. If no pivot integer is found, I’ll return -1.

Complexity

  • Time complexity: O(n)

  • Space complexity: O(1)

Code

class Solution:
    def pivotInteger(self, n: int) -> int:
        curr_sum = 0
        total_sum = sum(range(n + 1))
        for i in range(1, n + 1):
            curr_sum += i
            if total_sum - curr_sum + i == curr_sum:
                return i
        return -1

Editorial Solution

Approach 2: Two Pointer

class Solution:
    def pivotInteger(self, n: int) -> int:
        left_value = 1
        right_value = n
        sum_left = left_value
        sum_right = right_value

        if n == 1:
            return n

        # Iterate until the pointers meet
        while left_value < right_value:
            # Adjust sums and pointers based on comparison
            if sum_left < sum_right:
                sum_left += left_value + 1
                left_value += 1
            else:
                sum_right += right_value - 1
                right_value -= 1

            # Check for pivot condition
            if sum_left == sum_right and left_value + 1 == right_value - 1:
                return left_value + 1

        return -1  # Return -1 if no pivot is found
  • Time complexity: O(n)
  • Space complexity: O(1)
class Solution:
    def pivotInteger(self, n: int) -> int:
        # Initialize left and right pointers for binary search
        left, right = 1, n

        # Calculate the total sum of the sequence
        total_sum = n * (n + 1) // 2

        # Binary search for the pivot point
        while left < right:
            # Calculate the mid-point
            mid = (left + right) // 2

            # Check if the difference between the square of mid and the total sum is negative
            if mid * mid - total_sum < 0:
                left = mid + 1  # Adjust the left bound if the sum is smaller
            else:
                right = mid  # Adjust the right bound if the sum is equal or greater

        # Check if the square of the left pointer minus the total sum is zero
        if left * left - total_sum == 0:
            return left
        else:
            return -1
  • Time complexity: O(log n)
  • Space complexity: O(1)

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