> For the complete documentation index, see [llms.txt](https://hao-fu-1.gitbook.io/oj/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hao-fu-1.gitbook.io/oj/binary_tree_and_divide_conquer/order-traversal/742.-closest-leaf-in-a-binary-tree.md).
# 742. Closest Leaf in a Binary Tree
Given a binary tree **where every node has a unique value**, and a target key `k`, find the value of the nearest leaf node to target `k` in the tree.
Here, nearest to a leaf means the least number of edges travelled on the binary tree to reach any leaf of the tree. Also, a node is called a leaf if it has no children.
In the following examples, the input tree is represented in flattened form row by row. The actual `root` tree given will be a TreeNode object.
**Example 1:**
```
Input:
root = [1, 3, 2], k = 1
Diagram of binary tree:
1
/ \
3 2
Output: 2 (or 3)
Explanation: Either 2 or 3 is the nearest leaf node to the target of 1.
```
**Example 2:**
```
Input:
root = [1], k = 1
Output: 1
Explanation: The nearest leaf node is the root node itself.
```
**Example 3:**
```
Input:
root = [1,2,3,4,null,null,null,5,null,6], k = 2
Diagram of binary tree:
1
/ \
2 3
/
4
/
5
/
6
Output: 3
Explanation: The leaf node with value 3 (and not the leaf node with value 6) is nearest to the node with value 2.
```
**Note:**
1. `root` represents a binary tree with at least `1` node and at most `1000` nodes.
2. Every node has a unique `node.val` in range `[1, 1000]`.
3. There exists some node in the given binary tree for which `node.val == k`.
二叉树每个节点的值是唯一的，找离结点值为K的最近的叶结点。遍历树生成图，从K结点处做BFS直到遇到一个叶结点。
```cpp
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* dfs(TreeNode *cur, unordered_map &edges, int k) {
if (cur == nullptr) return nullptr;
if (cur->val == k) return cur;
if (cur->left != nullptr) {
edges[cur->left] = cur;
auto l = dfs(cur->left, edges, k);
if (l != nullptr) return l;
}
if (cur->right != nullptr) {
edges[cur->right] = cur;
auto r = dfs(cur->right, edges, k);
if (r != nullptr) return r;
}
return nullptr;
}
int findClosestLeaf(TreeNode* root, int k) {
unordered_map edges;
auto k_node = dfs(root, edges, k);
if (k_node == nullptr) return -1;
queue q;
q.push(k_node);
unordered_set visited;
while (!q.empty()) {
for (int size = q.size(); size > 0; --size) {
auto t = q.front(); q.pop();
if (t->left == nullptr && t->right == nullptr) return t->val;
if (visited.count(t)) continue;
visited.insert(t);
if (t->left != nullptr) q.push(t->left);
if (t->right != nullptr) q.push(t->right);
if (edges.count(t)) q.push(edges[t]);
}
}
return -1;
}
};
```
---
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