Blind 75 in Java: Collections, Idioms and Pitfalls by Category

Java solutions to the Blind 75 are longer than Python's but very readable, and Java's fixed 32-bit integers make the bit manipulation problems behave exactly as written. The traps are elsewhere: comparators that only work on boxed arrays, an ArrayDeque that rejects nulls and overflow in comparisons. This page goes through the list's categories with the APIs each needs, the problems where Java needs care and a six-week order. For the list, see our Blind 75 guide; for language questions, our Java interview questions.

The list by category

Blind 75 has 19 easy, 49 medium and 7 hard problems, six of them Premium. Counts were read from each list's own site data in November 2026. Lists are edited from time to time, so numbers can drift.

Idioms that cut typing

• Grouping: map.computeIfAbsent(key, k -> new ArrayList<>()).add(s) replaces four lines of null checks in Group Anagrams.
• Anagram keys: sort a char[] and use String.valueOf(chars) as the key, or build a key from an int[26] count.
• Counting: map.merge(x, 1, Integer::sum) for Top K Frequent Elements, then bucket by frequency.
• Interval sorting: Arrays.sort(intervals, (a, b) -> Integer.compare(a[0], b[0])). Comparators work on object arrays such as int[][] but not on a plain int[], which Arrays.sort only sorts ascending.
• Coin Change: Arrays.fill(dp, amount + 1) as an 'infinity' that cannot overflow when you add one.
• Grid keys: a record Cell(int r, int c) {} gets equals and hashCode for free for visited sets; r * cols + c as an int is faster.

Problems where Java needs care

• Merge k Sorted Lists: new PriorityQueue<ListNode>((a, b) -> Integer.compare(a.val, b.val)); a subtraction comparator such as a.val - b.val overflows on extreme values.
• Binary Tree Level Order Traversal and Serialize and Deserialize: ArrayDeque throws a NullPointerException on null elements. Skip null children, or use LinkedList when you must enqueue nulls.
• Reverse Bits and Number of 1 Bits: the input is a signed int; shift with >>> so the sign bit does not spread. Integer.reverse and Integer.bitCount exist, but interviewers usually want the loop.
• Sum of Two Integers: the carry loop with ^, & and << works as written because Java integers wrap at 32 bits.
• Valid Palindrome: Character.isLetterOrDigit and Character.toLowerCase with two indexes avoid building a cleaned copy.
• Find Median from Data Stream: a max-heap with Collections.reverseOrder() and a min-heap; rebalance so sizes differ by at most one.
• Meeting Rooms II (Premium): sort start and end times into two int[] arrays and sweep with two pointers.

A six-week order

1. Week 1: arrays and hashing, two pointers and sliding window.
2. Week 2: stack, binary search and linked lists.
3. Week 3: trees and tries.
4. Week 4: heap, backtracking and graphs.
5. Week 5: dynamic programming, all twelve problems.
6. Week 6: greedy, intervals, math and bits, then redo the problems you failed.

LeetCode runs Java on OpenJDK 25, as LeetCode's help page on language environments listed it on November 11, 2026, so records and var are available there; check the version in your interview editor. Next steps after Blind 75: NeetCode 150 in Java for 75 more problems, or the ranked Grind 75 guide.

Turning a list into interview skill

Any list only helps if the practice transfers to problems you have not seen. These habits make the difference:

1. Time-box every attempt: about 25 minutes for a medium before you read a solution.
2. Keep a log with one line per problem: the pattern, the key idea and the complexity.
3. Redo failed problems after three days, a week and three weeks, without looking at your old code.
4. Explain your approach out loud before and while you code, as you would to an interviewer.
5. In the last two weeks, switch to unseen problems without topic labels.

The patterns most lists are built on are explained in array and string patterns and tree, graph and recursion patterns.

Where CoPilot Interview fits

Finishing a list is not the same as passing an interview: interviewers score how you explain your approach, your trade-offs and your complexity out loud. CoPilot Interview is an AI interview assistant; its free AI mock interview asks coding questions, listens to your spoken explanation and scores it out of ten with a model answer, which is the part a problem list cannot train. The free plan gives 15 minutes a day of live help in sessions of up to 15 minutes, with AI mock interviews included. Online assessments and live coding rounds usually forbid outside help, so practice now and follow your employer's rules on the day.

Disclosure: CoPilot Interview is our product, so weigh what we say about it accordingly.

Frequently asked questions

Is Java a good language for Blind 75?

Yes. Solutions are longer than in Python but readable, and Java's 32-bit integers make the bit problems straightforward.

Why does my Java BFS throw a NullPointerException?

ArrayDeque does not accept null elements. Skip null children before adding them, or use LinkedList if you need to queue nulls.

How do I sort intervals in Java?

Arrays.sort(intervals, (a, b) -> Integer.compare(a[0], b[0])). Avoid a[0] - b[0], which can overflow.

Which Java version does LeetCode use?

OpenJDK 25, as LeetCode's help page listed it in November 2026.