1061. Lexicographically Smallest Equivalent String

You are given two strings of the same length s1 and s2 and a string baseStr.

We say s1[i] and s2[i] are equivalent characters.

• For example, if s1 = “abc” and s2 = “cde”, then we have ‘a’ == ‘c’, ‘b’ == ‘d’, and ‘c’ == ‘e’.

Equivalent characters follow the usual rules of any equivalence relation:

• Reflexivity: ‘a’ == ‘a’.
• Symmetry: ‘a’ == ‘b’ implies ‘b’ == ‘a’.
• Transitivity: ‘a’ == ‘b’ and ‘b’ == ‘c’ implies ‘a’ == ‘c’.

For example, given the equivalency information from s1 = “abc” and s2 = “cde”, “acd” and “aab” are equivalent strings of baseStr = “eed”, and “aab” is the lexicographically smallest equivalent string of baseStr.

Return the lexicographically smallest equivalent string of baseStr by using the equivalency information from s1 and s2.

1639. Number of Ways to Form a Target String Given a Dictionary

You are given a list of strings of the same length words and a string target.

Your task is to form target using the given words under the following rules:

• target should be formed from left to right.
• To form the ith character (0-indexed) of target, you can choose the kth character of the jth string in words if target[i] = words[j][k].
• Once you use the kth character of the jth string of words, you can no longer use the xth character of any string in words where x <= k. In other words, all characters to the left of or at index k become unusuable for every string.
• Repeat the process until you form the string target.

Notice that you can use multiple characters from the same string in words provided the conditions above are met.

Return the number of ways to form target from words. Since the answer may be too large, return it modulo 109 + 7.

202. Happy Number

Write an algorithm to determine if a number n is happy.

A happy number is a number defined by the following process:

• Starting with any positive integer, replace the number by the sum of the squares of its digits.
• Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
• Those numbers for which this process ends in 1 are happy.

Return true if n is a happy number, and false if not.

1790. Check if One String Swap Can Make Strings Equal

You are given two strings s1 and s2 of equal length. A string swap is an operation where you choose two indices in a string (not necessarily different) and swap the characters at these indices.

Return true if it is possible to make both strings equal by performing at most one string swap on exactly one of the strings. Otherwise, return false.

1502. Can Make Arithmetic Progression From Sequence

A sequence of numbers is called an arithmetic progression if the difference between any two consecutive elements is the same.

Given an array of numbers arr, return true if the array can be rearranged to form an arithmetic progression. Otherwise, return false.

1004. Max Consecutive Ones III

Given a binary array nums and an integer k, return the maximum number of consecutive 1’s in the array if you can flip at most k 0’s.

228. Summary Ranges

You are given a sorted unique integer array nums.

Return the smallest sorted list of ranges that cover all the numbers in the array exactly. That is, each element of nums is covered by exactly one of the ranges, and there is no integer x such that x is in one of the ranges but not in nums.

Each range [a,b] in the list should be output as:

• “a->b” if a != b
• “a” if a == b

1220. Count Vowels Permutation

Given an integer n, your task is to count how many strings of length n can be formed under the following rules:

• Each character is a lower case vowel (‘a’, ‘e’, ‘i’, ‘o’, ‘u’)
• Each vowel ‘a’ may only be followed by an ‘e’.
• Each vowel ‘e’ may only be followed by an ‘a’ or an ‘i’.
• Each vowel ‘i’ ‘i’.
• Each vowel ‘o’ may only be followed by an ‘i’ or a ‘u’.
• Each vowel ‘u’ may only be followed by an ‘a’.

Since the answer may be too large, return it modulo 10^9 + 7.

766. Toeplitz Matrix

Given an m x n matrix, return true if the matrix is Toeplitz. Otherwise, return false.

A matrix is Toeplitz if every diagonal from top-left to bottom-right has the same elements.

976. Largest Perimeter Triangle

Given an integer array nums, return the largest perimeter of a triangle with a non-zero area, formed from three of these lengths. If it is impossible to form any triangle of a non-zero area, return 0.