Closest Leaf in a Binary Tree

本文主要是介绍Closest Leaf in a Binary Tree，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

Input:
root = [1,2,3,4,null,null,null,5,null,6], k = 2
Diagram of binary tree:1/ \2   3/4/5/6Output: 3
Explanation: The leaf node with value 3 (and not the leaf node with value 6) is nearest to the node with value 2.

思路：把tree变换成图，建立图的过程，顺便把startNode找到；然后在图中做BFS，无向图，必须要有visited，否则会有死循环；

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public int findClosestLeaf(TreeNode root, int k) {HashMap<TreeNode, List<TreeNode>> graph = new HashMap<>();TreeNode[] targetNode = new TreeNode[1];buildGraph(graph, root, null, targetNode, k);Queue<TreeNode> queue = new LinkedList<>();HashSet<TreeNode> visited = new HashSet<>();queue.offer(targetNode[0]);visited.add(targetNode[0]);while(!queue.isEmpty()) {int size = queue.size();for(int i = 0; i < size; i++) {TreeNode node = queue.poll();if(node.left == null && node.right == null) {return node.val;}for(TreeNode neighbor: graph.get(node)) {if(!visited.contains(neighbor)) {visited.add(neighbor);queue.offer(neighbor);}}}}return -1;}private void buildGraph(HashMap<TreeNode, List<TreeNode>> graph,TreeNode root, TreeNode parent, TreeNode[] targetNode, int target) {if(root == null) {return;}if(root.val == target) {targetNode[0] = root;}graph.putIfAbsent(root, new ArrayList<TreeNode>());if(root.left != null) {graph.get(root).add(root.left);buildGraph(graph, root.left, root, targetNode, target);}if(root.right != null) {graph.get(root).add(root.right);buildGraph(graph, root.right, root, targetNode, target);}if(parent != null) {graph.get(root).add(parent);}}
}

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