Swift 每週五題 LeetCode(7)

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46. Permutations (Medium)

題目會給一組陣列,要列出陣列內數字的排列組合
Example 1:

Input: nums = [1,2,3]
Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

Permutations - LeetCode

func permute(_ nums: [Int]) -> [[Int]] {
    var result : [[Int]] = []
    var numeros = nums
    
    // Base case
    if (nums.count == 1){
        return [nums]
    }
    
    var perms : [[Int]] = []
    
    for _ in 0..<nums.count {
        let firstElement = numeros.first!
        numeros.removeFirst()
        // [2, 3] -> perms remove 2 -> 3
        perms = permute(numeros) // 遞迴 [[3]] // [[2]] //
        
        for i in 0..<perms.count {
            perms[i].append(firstElement) // [[3,2]]  // [[2,3]]
        }
        
        numeros.append(firstElement) // [3, 2]
        
        // Add permutiations to result array
        for i in 0..<perms.count {
            result.append(perms[i])
        }
        
    }
    
    return result
}

48. Rotate Image (Medium)

https://leetcode.com/problems/rotate-image/
Example 1:

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

請把圖向右轉90度：Ｐ（把每組陣列中第一個數拿出來）

func rotate(_ matrix: inout [[Int]]) {
    var result: [[Int]] = []
    var tempArr: [Int] = [] //暫時儲存的[]
    let arrCount = matrix.count - 1
    
    for _ in 0...arrCount {
        for k in 0...arrCount { //先把第一個移除,再加回來
            let arr = matrix[arrCount - k].removeFirst()
            tempArr.append(arr)
        }
        result.append(tempArr) //最後加到二維陣列中
        tempArr = []
    }
    matrix = result
}

49. Group Anagrams (Medium)

Given an array of strings strs, group the anagrams together. You can return the answer in any order.

An Anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.

Example 1:

Input: strs = ["eat","tea","tan","ate","nat","bat"]
Output: [["bat"],["nat","tan"],["ate","eat","tea"]]

Group Anagrams - LeetCode

func groupAnagrams(_ strs: [String]) -> [[String]] {
    var result = [[String]]()
    var records = [String]()
    var flag = false //記錄狀態是否改變
    
    for str in strs {
        let recordItem = String(str.sorted())
        flag = false
        for i in 0..<records.count {
            if records[i] == recordItem {
                result[i].append(str)
                flag = true
                break
            }
        }
        if flag == false {
            result.append([str])
            records.append(recordItem)
        }
    }
    return result
}

50. Pow(x, n) (Medium)

https://leetcode.com/problems/powx-n/
這題有個偷吃步的解法ＸＤ

func myPow(_ x: Double, _ n: Int) -> Double {
    return pow(x, Double(n)) // 第一個解法ＸＤ
    var n = n // 第二個解法
    var x = x
    if n == 0 {
        return 1
    }
    if n < 0 {
        n = -n
        x = 1/x
    }
    var ans: Double = 1
    while n > 0 {
        if n & 1 != 0 {
            ans = ans * x
        }
        x = x * x
        n = n >> 1
    }
    return ans
}

53. Maximum Subarray (Easy)

Maximum Subarray - LeetCode

找出總和最大的連續子數組

func maxSubArray(_ nums: [Int]) -> Int {
    let nums = nums
    var sum = nums[0]
    var current = nums[0]
    
    for i in 1..<nums.count {
        current = max(current + nums[i], nums[i])
        sum = max(current, sum)
    }
    
    return sum
}