Pattern concept

Step 1 of 11

Use ← → arrow keys

Code snippets

# Find two numbers in the list nums that sum up to target.
def two_sum(nums, target):

  a = 0  # Initialize a pointer at beginning of list.
  b = len(nums) - 1  # Initialize another pointer at end of list.

  while a < b:
      current_sum = nums[a] + nums[b]
      
      if current_sum == target:
          return [a, b]  # Return the pair that sums up to target.
      elif current_sum < target:
          a += 1  # We are under target so increase the sum.
      else:
          b -= 1  # We are over target so decrease the sum.

  return None  # We never found a pair that sums up to target.

How to identify

Does the problem involve any of these?

• Searching for pairs or triplets of elements that satisfy certain conditions
  • Sum of Three: Find triplets that sum up to a target.
  • Best Time to Buy and Sell Stock: Find a pair of elements with max profit.
• Maintaining two pointers to scan a list simultaneously from both ends
  • Valid Palindrome: Check whether a string is a valid palindrome.
  • Valid Palindrome II: Check whether a string is almost a valid palindrome.
  • Trapping Rain Water: Scan the list to calculate the amount of rainwater that can be trapped.
• Maintaining a sliding window or range within the array or list (covered later)
• Manipulating pointers to nodes in a linked list or tree (covered later)
• merging or comparing two sorted lists (covered later)