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花花酱 LeetCode 2089. Find Target Indices After Sorting Array

By zxi on November 28, 2021

You are given a 0-indexed integer array nums and a target element target.

A target index is an index i such that nums[i] == target.

Return a list of the target indices of nums after sorting nums in non-decreasing order. If there are no target indices, return an empty list. The returned list must be sorted in increasing order.

Example 1:

Input: nums = [1,2,5,2,3], target = 2
Output: [1,2]
Explanation: After sorting, nums is [1,2,2,3,5].
The indices where nums[i] == 2 are 1 and 2.

Example 2:

Input: nums = [1,2,5,2,3], target = 3
Output: [3]
Explanation: After sorting, nums is [1,2,2,3,5].
The index where nums[i] == 3 is 3.

Example 3:

Input: nums = [1,2,5,2,3], target = 5
Output: [4]
Explanation: After sorting, nums is [1,2,2,3,5].
The index where nums[i] == 5 is 4.

Example 4:

Input: nums = [1,2,5,2,3], target = 4
Output: []
Explanation: There are no elements in nums with value 4.

Constraints:

  • 1 <= nums.length <= 100
  • 1 <= nums[i], target <= 100

Solution: Sorting

Time complexity: O(nlogn)
Space complexity: O(1)

C++

花花酱 LeetCode 137. Single Number II

By zxi on November 27, 2021

Given an integer array nums where every element appears three times except for one, which appears exactly once. Find the single element and return it.

You must implement a solution with a linear runtime complexity and use only constant extra space.

Example 1:

Input: nums = [2,2,3,2]
Output: 3

Example 2:

Input: nums = [0,1,0,1,0,1,99]
Output: 99

Constraints:

  • 1 <= nums.length <= 3 * 104
  • -231 <= nums[i] <= 231 - 1
  • Each element in nums appears exactly three times except for one element which appears once.

Solution: Bit by bit

Since every number appears three times, the i-th bit must be a factor of 3, if not, that bit belongs to the single number.

Time complexity: O(32n)
Space complexity: O(1)

C++

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花花酱 LeetCode 136. Single Number

By zxi on November 27, 2021

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one.

You must implement a solution with a linear runtime complexity and use only constant extra space.

Example 1:

Input: nums = [2,2,1]
Output: 1

Example 2:

Input: nums = [4,1,2,1,2]
Output: 4

Example 3:

Input: nums = [1]
Output: 1

Constraints:

  • 1 <= nums.length <= 3 * 104
  • -3 * 104 <= nums[i] <= 3 * 104
  • Each element in the array appears twice except for one element which appears only once.

Solution: XOR

single_number ^ a ^ b ^ c ^ … ^ a ^ b ^ c … = single_number

Time complexity: O(n)
Space complexity: O(1)

C++

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花花酱 LeetCode 119. Pascal’s Triangle II

By zxi on November 27, 2021

Given an integer rowIndex, return the rowIndexth (0-indexed) row of the Pascal’s triangle.

In Pascal’s triangle, each number is the sum of the two numbers directly above it as shown:

Example 1:

Input: rowIndex = 3
Output: [1,3,3,1]

Example 2:

Input: rowIndex = 0
Output: [1]

Example 3:

Input: rowIndex = 1
Output: [1,1]

Constraints:

  • 0 <= rowIndex <= 33

Follow up: Could you optimize your algorithm to use only O(rowIndex) extra space?

Solution: Remember last row

Time complexity: O(n2)
Space complexity: O(n)

C++

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花花酱 LeetCode 118. Pascal’s Triangle

By zxi on November 27, 2021

Given an integer numRows, return the first numRows of Pascal’s triangle.

In Pascal’s triangle, each number is the sum of the two numbers directly above it as shown:

Example 1:

Input: numRows = 5
Output: [[1],[1,1],[1,2,1],[1,3,3,1],[1,4,6,4,1]]

Example 2:

Input: numRows = 1
Output: [[1]]

Constraints:

  • 1 <= numRows <= 30

Solution: Simulation

Time complexity: O(n2)
Space complexity: O(n2)

C++

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