kth_smallest_element_bst
kth_smallest_element_bst.cpp 源码
// 二叉搜索树中第k小的元素
#include "stack"
#include "queue"
using namespace std;
struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode() {}
TreeNode(int val) : val(val) {}
TreeNode(int val, TreeNode *left, TreeNode *right) : val(val), left(left), right(right) {}
};
class Solution {
public:
// 中序遍历，取第k个
int kthSmallest1(TreeNode* root, int k) {
vector<int> vals;
helper(root, vals);
return vals[k - 1];
}
void helper(TreeNode* root, vector<int>& vals) {
if (!root) {
return;
}
helper(root->left, vals);
vals.push_back(root->val);
helper(root->right, vals);
}
// 中序遍历优化，取到第k个时结束
int kthSmallest2(TreeNode* root, int k) {
stack<TreeNode*> s;
TreeNode* cur = root;
while (!s.empty() || cur) {
while (cur) {
s.push(cur);
cur = cur->left;
}
TreeNode* node = s.top();
s.pop();
if (--k == 0) {
return node->val;
}
cur = node->right;
}
return 0;
}
};
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