Given a binary tree where every node has a unique value, and a target keyk, find the value of the nearest leaf node to targetkin the tree.
Here,nearestto 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 aleafif it has no children.
In the following examples, the input tree is represented in flattened form row by row. The actualroottree given will be a TreeNode object.
Example
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
DFS寻找目标并构建,backward map作为parent node
BFS上下进行遍历,寻找下一层的叶子结点
Code
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */classSolution {publicintfindClosestLeaf(TreeNode root,int k) {Map<TreeNode,TreeNode> backMap =newHashMap<>();TreeNode node =dfs(root, k, backMap);returnbfs(node, backMap); }privateintbfs(TreeNode node,Map<TreeNode,TreeNode> backMap) {Queue<TreeNode> q =newLinkedList<>();Set<TreeNode> set =newHashSet<>();q.offer(node);set.add(node);while (!q.isEmpty()) {int size =q.size();for (int i =0; i < size; i++) {TreeNode curr =q.poll();if (curr.left==null&&curr.right==null) {returncurr.val; }if (curr.left!=null&&!set.contains(curr.left)) {q.offer(curr.left);set.add(curr.left); }if (curr.right!=null&&!set.contains(curr.right)) {q.offer(curr.right);set.add(curr.right); }if (backMap.get(curr) !=null&&!set.contains(backMap.get(curr))) {q.offer(backMap.get(curr));set.add(backMap.get(curr)); } } }return-1; }privateTreeNodedfs(TreeNode root,int k,Map<TreeNode,TreeNode> backMap) {if (root.val== k) {return root; }if (root.left!=null) {backMap.put(root.left, root);TreeNode left =dfs(root.left, k, backMap);if (left !=null) {return left; } }if (root.right!=null) {backMap.put(root.right, root);TreeNode right =dfs(root.right, k, backMap);if (right !=null) {return right; } }returnnull; }}