剑指Offer(54)二叉搜索树的第K大结点

/*
struct TreeNode {
int val;
struct TreeNode *left;
struct TreeNode *right;
TreeNode(int x) :
val(x), left(NULL), right(NULL) {
}
};
*/
class Solution {
public:
TreeNode* KthNode(TreeNode* pRoot, int k)
{
if(pRoot==nullptr||k==0)//空值检测
return nullptr;
return KthCore(pRoot,k);//防止每次递归都进行空值检测，并增强函数的复用性
}
int count=0;
TreeNode* KthCore(TreeNode* pRoot,int k)
{
TreeNode* target=nullptr;//目标的返回值
//遍历左子树
if(pRoot->left!=nullptr)
{
target=KthCore(pRoot->left,k);
}
count++;
if(target==nullptr)
{
if(count==k)
{
target=pRoot;
return target;
}
}
//遍历右子树
if(target==nullptr&&pRoot->right!=nullptr)
{
target=KthCore(pRoot->right,k);
}
return target;
}
};