劍指offer:二叉搜尋樹的第k個結點
阿新 • • 發佈:2019-01-28
題目描述
給定一顆二叉搜尋樹,請找出其中的第k大的結點。例如, 5 / \ 3 7 /\ /\ 2 4 6 8 中,按結點數值大小順序第三個結點的值為4。
遞迴
/*
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, unsigned int k)
{
if (pRoot == NULL || k == 0){
return NULL;
}
return helper(pRoot, k);
}
private:
TreeNode* helper(TreeNode* pRoot, unsigned int& k){
TreeNode* res = NULL;
if (pRoot->left){
//
res= helper(pRoot->left, k);
}
if (pRoot){
if (res == NULL){
if (k == 1){
res = pRoot;
}
k--;
}
}
//res==NULL就不用再遞迴
if (res==NULL && pRoot->right!=NULL){
//
res=helper(pRoot->right, k);
}
return res;
}
};
非遞迴
/*
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, unsigned int k)
{
if (pRoot == NULL || k == 0){
return NULL;
}
stack<TreeNode*> stk;
TreeNode* p = pRoot;
int index = 0;
TreeNode* res=NULL;
while (p != NULL || !stk.empty()){
while (p){
stk.push(p);
p = p->left;
}
//這裡是if,不是while
if (!stk.empty()){
p = stk.top();
stk.pop();
index++;
if (index == k){
res = p;
break;
}
//不用判斷p->right是否為空
p = p->right;
}
}
return res;
}
};