DSA — Techniques

Two Pointer

Use two indices moving toward each other or in the same direction to solve pair/subarray problems in O(n) instead of O(n²).

— min read
O(n)Time
O(1)Space
SortedArrays
PairProblems
✓Slide+Shrink
// TWO SUM II — SORTED ARRAY
Find two numbers that sum to target
How It Works
Python Code
Use Cases
Quiz
Practice
01
Sort
Array must be sorted (or use hash map for unsorted).
arr.sort()
02
lo=0, hi=n-1
Start pointers at opposite ends.
lo, hi = 0, len(arr)-1
03
Check Sum
If sum too small → lo++. Too large → hi--.
if total < target: lo += 1
04
Found!
When sum equals target — return both indices.
return [lo, hi]
python 
def two_sum_sorted(arr, target):
    lo, hi = 0, len(arr) - 1
    while lo < hi:
        total = arr[lo] + arr[hi]
        if   total == target: return [lo, hi]
        elif total < target:  lo += 1
        else:                hi -= 1
    return []
Opposite ends: Two Sum, 3-Sum, Container with most water, Trapping rain water.

Same direction: Remove duplicates, sliding window, fast-slow pointer (cycle detection).
Two Sum 3-Sum Trapping Rain Water Container With Most Water Valid Palindrome Remove Duplicates
Two Pointer reduces O(n²) to O(n) because:
Progress 0 / 6 solved
6 problems · 5 PBC · 1 Startup
#001 Two Sum II (Sorted) Google PBC →
#007 3 Sum Amazon, Adobe PBC →
#008 Trapping Rain Water Google PBC →
#010 Container With Most Water Google, Meta PBC →
#028 Valid Palindrome Amazon PBC →
#017 Merge Sorted Arrays Cure.fit STARTUP →
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