LintCode 排列序号题目分析代码

public class Solution {
    /**
     * @param A an integer array
     * @return a long integer
     */
    public long permutationIndex(int[] permutation) {
        // Write your code here
        long index = 0;
  long position = 2;// position 1 is paired with factor 0 and so is skipped
  long factor = 1;
  for (int p = permutation.length - 2; p >= 0; p--) {
    long successors = 0;
    for (int q = p + 1; q < permutation.length; q++) {
      if (permutation[p] > permutation[q]) {
        successors++;
      }
    }
    index += (successors * factor);
    factor *= position;
    position++;
  }
  return index+1;
    }
}