我把解决GenomicRangeQuery任务的代码从Java重写到Swift。Jave中的代码得到了100/100的分数，但是Swift中的代码没有通过所有的性能测试。我试图理解为什么代码中的逻辑是相同的。我想知道为什么Swift代码执行这么长时间。我是否在我的Swift代码中使用了一些我不知道的非常慢的部分。请看一下从here复制的Java代码。

class Solution {
  public int[] solveGenomicRange(String S, int[] P, int[] Q) {
    //used jagged array to hold the prefix sums of each A, C and G genoms
    //we don't need to get prefix sums of T, you will see why.
    int[][] genoms = new int[3][S.length()+1];
    //if the char is found in the index i, then we set it to be 1 else they are 0
    // 3 short values are needed for this reason
    short a, c, g;
    for (int i=0; i<S.length(); i++) {
      a = 0; c = 0; g = 0;
      if ('A' == (S.charAt(i))) {
        a=1;
      }
      if ('C' == (S.charAt(i))) {
        c=1;
      }
      if ('G' == (S.charAt(i))) {
        g=1;
      }
      //here we calculate prefix sums. To learn what's prefix sums look at here https://codility.com/media/train/3-PrefixSums.pdf
      genoms[0][i+1] = genoms[0][i] + a;
      genoms[1][i+1] = genoms[1][i] + c;
      genoms[2][i+1] = genoms[2][i] + g;
    }

    int[] result = new int[P.length];
    //here we go through the provided P[] and Q[] arrays as intervals
    for (int i=0; i<P.length; i++) {
      int fromIndex = P[i];
      //we need to add 1 to Q[i],
      //because our genoms[0][0], genoms[1][0] and genoms[2][0]
      //have 0 values by default, look above genoms[0][i+1] = genoms[0][i] + a;
      int toIndex = Q[i]+1;
      if (genoms[0][toIndex] - genoms[0][fromIndex] > 0) {
        result[i] = 1;
      } else if (genoms[1][toIndex] - genoms[1][fromIndex] > 0) {
        result[i] = 2;
      } else if (genoms[2][toIndex] - genoms[2][fromIndex] > 0) {
        result[i] = 3;
      } else {
        result[i] = 4;
      }
    }
    return result;
  }
}

这里同样的代码被重写为Swift 2.1

public func solution(inout S:String, inout _ P:[Int], inout _ Q:[Int]) -> [Int] {
  let len = S.characters.count

  //used jagged array to hold the prefix sums of each A, C and G genoms
  //we don't need to get prefix sums of T, you will see why.
  var genoms = [[Int]](count: 3, repeatedValue: [Int](count: len+1, repeatedValue: 0))

  //if the char is found in the index i, then we set it to be 1 else they are 0
  // 3 short values are needed for this reason
  var a,c,g:Int
  for i in 0..<len {
    a=0; c=0; g=0
    let char = S[S.startIndex.advancedBy(i)]
    switch char {
    case "A": a=1;
    case "C": c=1;
    case "G": g=1;
    default: ()
    }

    //here we calculate prefix sums. To learn what's prefix sums look at here https://codility.com/media/train/3-PrefixSums.pdf
    genoms[0][i+1] = genoms[0][i] + a
    genoms[1][i+1] = genoms[1][i] + c
    genoms[2][i+1] = genoms[2][i] + g

  }

  var result: [Int] = [Int](count: P.count, repeatedValue: 0)
  //here we go through the provided P[] and Q[] arrays as intervals
  for i in 0..<P.count {
    let fromIndex = P[i]
    //we need to add 1 to Q[i],
    //because our genoms[0][0], genoms[1][0] and genoms[2][0]
    //have 0 values by default, look above genoms[0][i+1] = genoms[0][i] + a;
    let toIndex = Q[i] + 1

    if (genoms[0][toIndex] - genoms[0][fromIndex] > 0) {
      result[i] = 1;
    } else if (genoms[1][toIndex] - genoms[1][fromIndex] > 0) {
      result[i] = 2;
    } else if (genoms[2][toIndex] - genoms[2][fromIndex] > 0) {
      result[i] = 3;
    } else {
      result[i] = 4;
    }
  }
  return result
}

有人知道为什么Java代码通过了所有的测试，而Swift代码却没有通过所有的性能测试吗？我想我碰到了Swift中的某个敏感的瓶颈，但我不知道在哪里。
如果有人没有意识到编码，这是任务的link。