排序算法(1) 2018-03-07

选择排序

//selection sort an array a[] with size n

void selectionSort(int a[], int n) {
  int global, temp;
  for(in i = 0; i < n-1; i++) {
    global = i;
    for (int j = i +1; j < n; j++) {
      if(a[j] < a[global]) {
        global = j;  //record the index of the smallest element
       }
    }
  temp = a[i];
  a[i] = a[global];
  a[global] = temp;
  }
}

time complexity O(n^2)

归并排序

ArrayList mergeSort(ArrayList a, int left, int right){
  ArrayList solution;
  if(left == right){
    solution.add(a.get[left]);
    return solution;
  }
  int mid = left + (right - left) / 2;
  ArrayList solution_left = mergeSort(a, left, mid);
  ArrayList solution_right = mergeSort(a, mid + 1, right);
  solution = combine(solution_left, solution_right);
  return solution;
}

ArrayList combine(ArrayList a1, ArrayList a2) {
  int ia = 0;
  int ib = 0;
  ArrayList solution;
  while (ia < a1.size() && ib < a2.size()) {
    if (a1.get(ia) < a2.get(ib)) {
    solution.add(a1.get(ia));
    }
    else {
    solution.add(a2.get(ib));
    }
  }
  while(ia < a1.size()){
   solution.add(a1.get(ia);
  }
  while(ib < a2.size(){
  solution.add(a2.get(ib));
  }
  return solution;
}