Java中的递归合并

Question

首先，最重要的是，对于任何和所有的回复，预先感谢，他们不胜感激。现在到这篇文章的目的;我在Java中执行合并排序时遇到问题。我得到一个空指针异常，但是，我无法弄清楚错误是什么，对我来说代码看起来没问题。以下是我迄今：

public List mSort(List l) { 

    if (l.size() <= 1) 
     return l; 

    List left = null; 
    List right = null; 
    int mid = l.size()/2; 


    for (int x = 0; x < mid; x++) 
     left.add(l.get(x)); 
    for (int x = 0; x >= mid; x++) 
     right.add(l.get(x)); 

    left = mSort(left); 
    right = mSort(right); 

    return merge(left, right); 
} 

public List merge(List left, List right) { 

    List r = null; 
    while (left.size() > 0 || right.size() > 0) { 
     if (left.size() > 0 && right.size() > 0) 
      if (left.get(0) <= right.get(0)) { 
       r.add(left.get(0)); 
       left.remove(0); 
      } 
      else { 
       r.add(right.get(0)); 
       right.remove(0); 
      } 
     else if (left.size() > 0) { 
      r.add(left.get(0)); 
      left.remove(0); 

     } 
     else if (right.size() > 0) { 
      r.add(right.get(0)); 
      right.remove(0); 
     } 
    } 
    return r; 


} 

...显然，指针异常从“对于”中的“mSort”方法声明，凡是能指出我的错误或指导我未来正确的方向非常感谢！

Answer 1

left和right为空，您不能添加到！

List left = new ArrayList(); 
List right = new ArrayList(); 

Answer 2

for (int x = 0; x < mid; x++) 
     left.add(l.get(x)); 
    for (int x = 0; x >= mid; x++) 
     right.add(l.get(x)); 

必须

int x = 0; 
for (; x < mid; x++) 
    left.add(l.get(x)); 
for (; x < l.size(); x++) 
    right.add(l.get(x));