742. Closest Leaf in a Binary Tree查找最近的叶子节点

［抄题］：

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.

[暴力解法]：

时间分析：

空间分析：

[优化后]：

时间分析：

空间分析：

［奇葩输出条件］：

［奇葩corner case］：

［思维问题］：

不知道还要找点，把路径存在hashmap中

［英文数据结构或算法，为什么不用别的数据结构或算法］：

［一句话思路］：

bfs的过程中，cur节点的左、右、map中存储的路径都要放进q

［输入量］：空： 正常情况：特大：特小：程序里处理到的特殊情况：异常情况（不合法不合理的输入）：

［画图］：

［一刷］：

1. dfs函数的作用是返回有效的 值为k的节点，所以结果是左右节点的时候也需要返回
2. 左右节点均为空的时候，再返回root.val的数值

［二刷］：

1. dfs函数中，先把左节点放进去，再返回整个的left

［三刷］：

［四刷］：

［五刷］：

[五分钟肉眼debug的结果]：

［总结］：

用map存路径map.put(root.left, root);，然后用dfs找到k

［复杂度］：Time complexity: O(n) Space complexity: O(n)

［算法思想：迭代/递归/分治/贪心］：

［关键模板化代码］：

［其他解法］：

［Follow Up］：

［LC给出的题目变变变］：

[代码风格] ：

[是否头一次写此类driver funcion的代码] ：

[潜台词] ：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int findClosestLeaf(TreeNode root, int k) {
        //corner case
        if (root == null) return -1;

        //initialiazation: map, q
        //put first node into q, add left, right, route
        HashMap<TreeNode, TreeNode> map = new HashMap<TreeNode, TreeNode>();
        PriorityQueue<TreeNode> q = new PriorityQueue<TreeNode>();
        TreeNode match = dfsTree(k, root, map);

        q.add(match);
        while (!q.isEmpty()) {
            TreeNode cur = q.poll();
            if (cur.left == null && cur.right == null) return cur.val;
            if (cur.left != null) q.add(cur.left);
            if (cur.right != null) q.add(cur.right);
            if (map.containsKey(cur)) {
                q.add(map.get(cur));
                map.remove(cur);
            }
        }

        return -1;
    }

    public TreeNode dfsTree(int k, TreeNode root, Map<TreeNode, TreeNode> map) {
        //corner case : null
        if (root == null) return null;

        //return if left & right is null
        if (root.val == k) return root; 

        //put left into map and return left
        if (root.left != null) {
            map.put(root.left, root);
            TreeNode left = dfsTree(k, root.left, map);
            if (left != null) return left;
        } 

        //put left into map and return left
        if (root.right != null) {
            map.put(root.right, root);
            TreeNode right = dfsTree(k, root.right, map);
            if (right != null) return right;
        }

        //return null
        return null;
    }
}