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;
    }
};