Deleting leaves from a binary tree

问题

public void deleteLeaves(BSTNode<T> p){                     //deletes leaves
    if(p.left != null){
      if(isLeaf(p.left))
        p.left = null;
    }
    else if(p.right != null){
      if(isLeaf(p.right))
        p.right = null;
    }
    else{
      deleteLeaves(p.right);
      deleteLeaves(p.left); 
    }
  }

I cannot seem to figure out as to why it is not correctly deleting the leaves. It seems to only delete the final leaf of the tree. Any advice that helps would greatly be appreciated.

回答1:

Your if-statements are a bit messed up; Only one of the branches will be taken. Think about what should happen if neighter p.left nor p.right is null.

If I understand your data structure correctly, I'd probably write it like this:

// If there is something to the left...
if (p.left != null)
    if (isLeaf(p.left))
        p.left = null;             // delete it if it's a leaf...
    else
        deleteLeaves(p.left);      // else recurse.

// If there's something to the right...
if (p.right != null)
    if (isLeaf(p.right))
        p.right = null;            // delete it if it's a leaf...
    else
        deleteLeaves(p.right);     // else recurse.

回答2:

At the moment, if p.left != null then it won't even check for p.right. The else-if should just be an if. Also, you're saying if both neighboring nodes are null, then call deleteLeaves() on them, which makes no sense cuz they're null. Restructure your code; @aioobe has a good suggestion

回答3:

To use recursion properly, it must be in the same kind of control statement. You shouldn't check the condition in an if, and then do the actual recursion in an else if or else.

来源：https://stackoverflow.com/questions/7839664/deleting-leaves-from-a-binary-tree