Two Pointers & Sliding Window — Complete Guide (Problems 101–160)

Series Introduction

Welcome to the Two Pointers & Sliding Window series — the second chapter of our MAANG DSA Roadmap. This section covers 60 problems (#101–#160) grouped by pattern, from easy warm-ups to Google Hard problems.

Previous Series: Arrays & Strings (Problems 1–100)

Why Two Pointers & Sliding Window?

These are among the most asked patterns at FAANG/MAANG. They convert naive O(n²) brute-force solutions into clean O(n) algorithms by maintaining a structure that avoids redundant work.

Pattern	Core Idea	Typical Complexity
Fixed Window	Slide a window of size k	O(n)
Variable Window	Shrink/expand based on condition	O(n)
Two Pointers (opposite ends)	Meet in middle	O(n)
Two Pointers (same direction)	Fast/slow or read/write	O(n)
at-most(k) − at-most(k−1)	Count "exactly k"	O(n)
Prefix Sum + Deque	Handle negatives	O(n)

The Mental Model

Fixed Window:
  [l -------- r]  → slide right, always size k
  Sum: s += a[r] - a[r-k]

Variable Window (expand-then-shrink):
  l=0
  for r in range(n):
      add a[r] to window
      while window invalid:
          remove a[l], l++
      ans = max(ans, r-l+1)

Two Pointers — Inward:
  l=0, r=n-1
  while l < r:
      if condition: l++
      else: r--

Two Pointers — Same Direction (read/write):
  write=0
  for read in range(n):
      if keep(a[read]):
          a[write++] = a[read]

Problem Index

🟢 Easy (101–110)

#	Problem	Pattern	Difficulty
101	Valid Palindrome	Two Ptr Inward	Easy
102	Reverse String	Two Ptr Swap	Easy
103	Squares of Sorted Array	Two Ptr L/R	Easy
104	Remove Element	Read/Write Ptr	Easy
105	Remove Duplicates (Sorted)	Write Ptr	Easy
106	Merge Sorted Arrays	3-Ptr from End	Easy
107	Is Subsequence	Two Ptr	Easy
108	Max Consecutive Ones III	Sliding Window	Easy
109	Diet Plan Performance	Fixed Window	Easy
110	Running Sum 1D	Prefix Sum	Easy

🟡 Medium (111–150)

#	Problem	Pattern	Difficulty
111	Longest Substring Without Repeating	Variable Window	Medium
112	Longest Repeating Char Replacement	Window + Freq	Medium
113	Permutation in String	Fixed Window	Medium
114	Fruit Into Baskets	K-Distinct Window	Medium
115	Max Erasure Value	HashSet Window	Medium
116	Minimum Size Subarray Sum	Variable Window	Medium
117	Subarrays Size K & Avg≥Threshold	Fixed Window	Medium
118	Count Subarrays Max=K	Counting Window	Medium
119	4Sum II	Two Sum on Halves	Medium
120	3Sum	Sort + Two Ptr	Medium
121	Two Sum II (Sorted)	Two Ptr	Medium
122	Container with Most Water	Greedy Two Ptr	Medium
123	Boats to Save People	Greedy Two Ptr	Medium
124	Partition Labels	Two Ptr + Last Occ	Medium
125	Subarray Product Less Than K	Sliding Window	Medium
126	Max Points from Cards	Fixed Window (n-k)	Medium
127	Min Ops to Reduce X to Zero	Longest Mid Subarray	Medium
128	Count Nice Subarrays	At-Most Trick	Medium
129	Binary Subarrays with Sum	At-Most Trick	Medium
130	Subarrays K Different Integers	Exactly-K Window	Medium
131	Frequency of Most Frequent Element	Sorted + Window	Medium
132	Replace Substring Balanced String	Two Ptr	Medium
133	Longest Subarray of 1s Deleting One	Variable Window	Medium
134	Min Flips for Alternating Binary	Sliding Window	Medium
135	Take K of Each Character	Window Max Middle	Medium
136	Max Vowels in Substring of Len K	Fixed Window	Medium
137	Grumpy Bookstore Owner	Fixed Window Bonus	Medium
138	Min Swaps to Group 1s Together	Fixed Window	Medium
139	Longest Mountain in Array	Two Ptr Expand	Medium
140	Longest Turbulent Subarray	Two Ptr/DP	Medium
141	Min Diff Highest-Lowest K Scores	Sorted Window	Medium
142	Get Equal Substrings Within Budget	Sliding Window	Medium
143	Subarrays with All Three Chars	Three Ptr	Medium
144	Subarray Sum Divisible by K	Prefix + Modulo	Medium
145	Find K Closest Elements	Binary Search + Two Ptr	Medium
146	Sort Vowels in a String	Two Ptr	Medium
147	Count Subarrays with Fixed Bounds	Sliding Window	Medium
148	Max Consecutive Ones (Flip)	Variable Window	Medium
149	Sliding Window Median	Two Heaps	Hard+
150	Number of Substrings with All 3 Chars	Three Ptr	Medium

🔴 Hard (151–160)

#	Problem	Pattern	Difficulty
151	Minimum Window Substring	have/need Counter	Hard
152	Sliding Window Maximum	Monotonic Deque	Hard
153	Shortest Subarray Sum≥K	Deque on Prefix	Hard
154	Minimum Window Subsequence	Two Ptr DP	Hard
155	Count Subarrays Median=K	Prefix Balance	Hard
156	Longest Substring Two Distinct	Variable Window	Hard
157	Substring Concatenation All Words	Multi-Window	Hard
158	Min Consecutive Cards to Pick Up	HashMap Window	Hard
159	Count Subarrays Score < K	Two Ptr	Hard
160	Two Pointers & Sliding Window Recap	All Patterns	Summary

Core Template Cheatsheet

# Fixed Window
def fixed_window(nums, k):
    s = sum(nums[:k])
    best = s
    for i in range(k, len(nums)):
        s += nums[i] - nums[i-k]
        best = max(best, s)
    return best

# Variable Window (shrink when invalid)
def variable_window(nums, condition):
    left = best = 0
    window_state = initial_state()
    for right, val in enumerate(nums):
        add(val, window_state)
        while not valid(window_state):
            remove(nums[left], window_state)
            left += 1
        best = max(best, right - left + 1)
    return best

# Exactly K = atMost(K) - atMost(K-1)
def exactly_k(nums, k):
    return at_most(nums, k) - at_most(nums, k-1)

# Two Pointer Inward
def two_pointer(arr):
    l, r = 0, len(arr) - 1
    while l < r:
        if condition(arr[l], arr[r]): l += 1
        else: r -= 1

Next in Series

➡️ Hashing & Maps (Problems 161–220)