深度优先搜索
- 思路:
- 二叉搜索树的特性,通过中序遍历得到有序序列,则遍历到第K个节点的时候即为结果;
- 使用栈通过深度优先遍历进行中序遍历:
- 先将节点和左子节点压栈;
- 然后栈顶上就是“最左”叶子节点;
- 然后通过栈回溯其父节点 p ,将 p 右子树压栈,重复上述过程;
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
int kthSmallest(TreeNode* root, int k) {
std::stack<TreeNode*> stack;
TreeNode* node = root;
while (node != nullptr || stack.size() > 0) {
while (node != nullptr) {
stack.push(node);
node = node->left;
}
node = stack.top();
stack.pop();
--k;
if (k == 0) {
break;
}
node = node->right;
}
return node->val;
}
};