[Swift]LeetCode338. 比特位计数 | Counting Bits

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Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1's in their binary representation and return them as an array.

Example 1:

Input: 2
Output: [0,1,1]

Example 2:

Input: 5
Output: [0,1,1,2,1,2]

Follow up:

• It is very easy to come up with a solution with run time O(n*sizeof(integer)). But can you do it in linear time O(n)/possibly in a single pass?
• Space complexity should be O(n).
• Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.

---

给定一个非负整数 num。对于 0 ≤ i ≤ num 范围中的每个数字 i ，计算其二进制数中的 1 的数目并将它们作为数组返回。

示例 1:

输入: 2
输出: [0,1,1]

示例 2:

输入: 5
输出: [0,1,1,2,1,2]

进阶:

• 给出时间复杂度为O(n*sizeof(integer))的解答非常容易。但你可以在线性时间O(n)内用一趟扫描做到吗？
• 要求算法的空间复杂度为O(n)。
• 你能进一步完善解法吗？要求在C++或任何其他语言中不使用任何内置函数（如 C++ 中的 __builtin_popcount）来执行此操作。

---

36ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        guard num > 0 else {
            return [0]
        }
        
        var result = [0]
        var i = 0
        var total = 1
        
        for j in 1...num {
            result.append(result[i] + 1)
            i += 1
            if i == total {
                i = 0
                total = j + 1
            }
        }
        return result
    }
}

---

40ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        if num == 0 {
            return [0]
        }
        var results = [Int]()
        results.append(0)
        for n in 1...num {
            if n%2 == 1 {
                results.append(results[n-1]+1)
            } else {
                results.append(results[n/2])
            }
        }
        return results
    }
}

---

44ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        guard num > 0 else {
            return [0]
        }

        var result = Array(repeating: 0, count: num + 1)
        result[1] = 1
        var loopCount = 2
        var index = 2
        while index <= num {
            for j in 0..<loopCount {
                if index > num {
                    break
                }

                result[index] = 1 + result[j]
                index += 1
            }

            loopCount *= 2
        }
        return result
    }
}

---

48ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        var a = [0]
        var b = [Int]()
        while (a.count + b.count) != num+1 {
            if a.count == b.count{
                a = a + b
                b = [Int]()
            }
            b.append(a[b.count]+1)
        }
        return a + b
    }
}

---

88ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        guard num > 0 else { return [0] }
        var dp = [Int](repeating: 0, count: num + 1)
        
        for i in 1...num {
            dp[i] = dp[i & (i - 1)] + 1
        }
        return dp
    }
}

---

108ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        var result = [Int]()
        if num < 0 {
            return result
        }
        result.append(0)
        if num == 0 {
            return result
        }
        for i in 1 ... num {
            print(i & 1)
            result.append(result[i >> 1] + (i & 1))
        }
        return result
    }
}

---

112ms

class Solution {
    func countBits(_ num: Int) -> [Int] {       
        if num == 0 { return [0] }
        var result = [0]
        var count = 1
        while true {
            for j in 0 ..< count {
                result.append(result[j] + 1)    
                if result.count == num + 1 {
                    return result
                }
            }
            count = result.count
        }
        return result
        
    }
}

---

116ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        if num == 0 {
            return [0]
        }
        var res = [Int].init()
        res.append(0)
        for n in 1...num {
            let count = res[n & (n - 1)] + 1
            res.append(count)
        }
        return res
    }
}

---

164ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        var b = [Int](repeating: 0, count: num + 1)
        if num == 0 { return [0] }
        if num == 1 { return [0,1] }
        for i in 1...num {
            b[i] = b[i >> 1] + i % 2
        }
        return b
    }
}

---

176ms

class Solution {
    func countBits(_ num: Int) -> [Int]
    {
        var res = [Int]()
        for i in 0...num
        {
            var n = i
            var count = 0
            while n > 0
            {
                if n&1 == 1
                {
                    count += 1
                }
                n = n >> 1
            }
            res.append(count)
        }
        return res
    }
}

---

156ms

class Solution {
    func countBits(_ num: Int) -> [Int] {
        guard num > 0 else { return [0] }
        var dp = [Int](repeating: 0, count: num + 1)
        
        for i in 1...num {
            dp[i] = dp[i / 2]
            if i % 2 == 1 {
                dp[i] += 1
            }
        }
        return dp
    }
}