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7 problems asked at Facebook.

Easy (2)

Top K Frequent Elements (Leetcode 347)

Arrays, Hashing, Heap...

Valid Anagram (leetcode 242)

Arrays, Hashing, Strings

Medium (2)

Group Anagrams (Leetcode 49)

Arrays, Strings, Sorting...

Longest Substring Without Repeating Characters (leetcode 3)

Sliding Window, Strings, Hashing

Hard (1)

Trapping Rain Water (LC 42)

Two Pointers, Prefix and Suffix Arrays, Stack

Not Specified (2)

Best Time to Buy and Sell Stock

Arrays, DP

Container With Most Water

Two Pointers