742. Closest Leaf in a Binary Tree

https://leetcode.com/problems/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。

/**
 * 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<TreeNode*, TreeNode*> &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<TreeNode*, TreeNode*> edges;
        auto k_node = dfs(root, edges, k);
        if (k_node == nullptr) return -1;
        queue<TreeNode*> q;
        q.push(k_node);
        unordered_set<TreeNode*> 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;
    }
};