[Swift]LeetCode118. 杨辉三角 | Pascal's Triangle

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Given a non-negative integer numRows, generate the first numRows of Pascal's triangle.

In Pascal's triangle, each number is the sum of the two numbers directly above it.

Example:

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

---

给定一个非负整数 numRows，生成杨辉三角的前 numRows 行。

在杨辉三角中，每个数是它左上方和右上方的数的和。

示例:

输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

---

class Solution {
    func generate(_ numRows: Int) -> [[Int]] {
        var res:[[Int]] =  [[Int]]()
        if numRows <= 0 {return res}
        var arr:[Int] = [Int]()
        arr.append(1)
        res.append(arr)
        
        for i in 1..<numRows
        {
            var temp:[Int] = [Int]() 
            temp.append(1)
            
            var count:Int = res[i - 1].count
            for j in 1..<count
            {
                temp.append(res[i-1][j]+res[i-1][j-1])
            }
            temp.append(1)
            res.append(temp)
        }
        return res
    }
}

---

8ms

class Solution {
    func generate(_ numRows: Int) -> [[Int]] {
    guard numRows > 0 else {
        return []
    }
    
    var temp = [[1]]
    for i in 1..<numRows {
        let last = temp[i - 1]
        var arr = [Int]()
        for j in 0...i {
            var n : Int
            if j == 0 {
                n = 1
            }else if j == i {
                n = last[j - 1]
            }else {
                n = last[j] + last[j - 1]
            }
            arr.append(n)
        }
        temp.append(arr)
    }
    
    return temp
    }
}

---

8ms

class Solution {
    func generate(_ numRows: Int) -> [[Int]] {
        if numRows == 0 { return [] }
        else if numRows == 1 { return [[1]] }
        else if numRows == 2 { return [[1], [1,1]] }
        
        var memo = [[1], [1,1]]
        
        for i in 1..<numRows-1 {
            let prevArr = memo[i]
            var arr = Array(repeating: 0, count: prevArr.count+1)
            arr[0] = 1
            arr[arr.count-1] = 1

            for j in 1..<arr.count-1 {
                arr[j] = prevArr[j-1]+prevArr[j]
            }
            
            memo.append(arr)
        }
        
        return memo
    }
}

---

12ms

class Solution {
    func generate(_ numRows: Int) -> [[Int]] {
        if numRows == 0 {
            return []
        }
        var result = [[1]]
        for i in 1...numRows {
            var numRow = [Int]()
            var lastNumRow = result[i - 1]
            for j in 0..<i {
                if j == 0 || j == i - 1 {
                    numRow.append(1)
                } else {
                    numRow.append(lastNumRow[j - 1] + lastNumRow[j])
                }
            }
            result.append(numRow)
        }
        result.remove(at: 0)
        return result
    }
}