merge sort二刷

推荐答案

public class Solution {
  public int[] mergeSort(int[] array) {
    // Write your solution here.
    if(array != null && array.length != 0) {
      int[] helper = new int[array.length];
      
      divide(array, helper, 0, array.length - 1);
    }
    
    return array;
  }
  
  private void divide(int[] array, int[] helper, int left, int right) {
    if(left == right) {
        return;
    }
    int mid = left + (right - left) / 2;
    divide(array, helper, left, mid);
    divide(array, helper, mid + 1, right);
    merge(array, helper, left, mid, right);
  }
  
  private void merge(int[] array, int[] helper, int left, int mid, int right) {
    for (int i = left; i <= right; i++) {
        helper[i] = array[i];
    }
    int leftIndex = left;
    int rightIndex = mid + 1;
    while (leftIndex <= mid && rightIndex <= right) {
      if(helper[leftIndex] <= helper[rightIndex]) {
          array[left++] = helper[leftIndex++];
      } else {
          array[left++] = helper[rightIndex++];
      }
    }
    
    while(leftIndex <= mid) {
        array[left++] = helper[leftIndex++];
    }
  }
}

二刷代码

public class Solution {
  public int[] mergeSort(int[] array) {
    // Write your solution here
    
    if (array != null && array.length != 0) {
        divide(array, 0, array.length - 1);
    }
    
    return array;
  }
  
  private void divide(int[] array, int left, int right) {
    if (left == right) {
        return;
    }
    
    int mid = left + (right - left) / 2;
    divide(array, left, mid);
    divide(array, mid + 1, right);
    merge(array, left, right, mid);
  }
  
  private void merge(int[] array, int left, int right, int mid) {
    // 此处是主要问题，第一是每次都费时创建了新helper对象，第二是每次都重复拷贝了所有的array元素
    // 知道helper应该只是用于存储当前merge所用到的元素，不过还没弄清应该如何设置拷贝的开始与结
    // 束端，需记住merge阶段传入的left即为左端，right即为右端，mid将从left到right的部分array
    // 划分为两部分，这被划分的两部分即为需要被merge排序的两个数组
    int[] helper = new int[array.length];
    for (int i = 0; i < array.length; i++) {
        helper[i] = array[i];
    }
    int i = left;
    int j = mid + 1;
    int k = left; // 不需要专门设置个k，可以直接使用left，用于对array进行操作
    
    while (i <= mid && j <= right) {
        if (helper[i] < helper[j]) {
          array[k] = helper[i];
        i++;
      } else {
          array[k] = helper[j];
        j++;
      }
      k++;
    }
    
    while (i <= mid) {
        array[k++] = helper[i++];
    }
    // 此处费时将剩余的helper的最右部分拷贝到array，需注意到merge是从左到右重写部分array，而且
    // helper就是部分array的拷贝，因此如果j指针未移动到right，j到right部分的helper和array是完全
    // 相同而且是已经排序好的，因此不需要拷贝剩余的helper的最右部分到array
    while (j <= right) {
        array[k++] = helper[j++];
    }
  }
}