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742. 二叉树最近的叶节点

English Version

题目描述

给定一个 每个结点的值互不相同 的二叉树，和一个目标整数值 k，返回 树中与目标值 k  最近的叶结点 。 

与叶结点最近 表示在二叉树中到达该叶节点需要行进的边数与到达其它叶结点相比最少。而且，当一个结点没有孩子结点时称其为叶结点。

 

示例 1：

输入：root = [1, 3, 2], k = 1
输出： 2
解释： 2 和 3 都是距离目标 1 最近的叶节点。

示例 2：

输入：root = [1], k = 1
输出：1
解释：最近的叶节点是根结点自身。

示例 3：

输入：root = [1,2,3,4,null,null,null,5,null,6], k = 2
输出：3
解释：值为 3（而不是值为 6）的叶节点是距离结点 2 的最近结点。

 

提示：

• 二叉树节点数在 [1, 1000] 范围内
• 1 <= Node.val <= 1000
• 每个节点值都 不同
• 给定的二叉树中有某个结点使得 node.val == k

解法

DFS 和 BFS。

先通过 DFS 构建图，然后 BFS 找距离值为 k 的结点最近的叶子结点。

Python3

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def findClosestLeaf(self, root: TreeNode, k: int) -> int:
        def dfs(root, p):
            if root:
                g[root].append(p)
                g[p].append(root)
                dfs(root.left, root)
                dfs(root.right, root)

        g = defaultdict(list)
        dfs(root, None)
        q = deque([node for node in g if node and node.val == k])
        seen = set()
        while q:
            node = q.popleft()
            seen.add(node)
            if node:
                if node.left is None and node.right is None:
                    return node.val
                for next in g[node]:
                    if next not in seen:
                        q.append(next)

Java

/**
 * 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 {
    private Map<TreeNode, List<TreeNode>> g;

    public int findClosestLeaf(TreeNode root, int k) {
        g = new HashMap<>();
        dfs(root, null);
        Deque<TreeNode> q = new LinkedList<>();
        for (Map.Entry<TreeNode, List<TreeNode>> entry : g.entrySet()) {
            if (entry.getKey() != null && entry.getKey().val == k) {
                q.offer(entry.getKey());
                break;
            }
        }
        Set<TreeNode> seen = new HashSet<>();
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            seen.add(node);
            if (node != null) {
                if (node.left == null && node.right == null) {
                    return node.val;
                }
                for (TreeNode next : g.get(node)) {
                    if (!seen.contains(next)) {
                        q.offer(next);
                    }
                }
            }
        }
        return 0;
    }

    private void dfs(TreeNode root, TreeNode p) {
        if (root != null) {
            g.computeIfAbsent(root, k -> new ArrayList<>()).add(p);
            g.computeIfAbsent(p, k -> new ArrayList<>()).add(root);
            dfs(root.left, root);
            dfs(root.right, root);
        }
    }
}

C++

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    unordered_map<TreeNode*, vector<TreeNode*>> g;

    int findClosestLeaf(TreeNode* root, int k) {
        dfs(root, nullptr);
        queue<TreeNode*> q;
        for (auto& e : g)
        {
            if (e.first && e.first->val == k)
            {
                q.push(e.first);
                break;
            }
        }
        unordered_set<TreeNode*> seen;
        while (!q.empty())
        {
            auto node = q.front();
            q.pop();
            seen.insert(node);
            if (node)
            {
                if (!node->left && !node->right) return node->val;
                for (auto next : g[node])
                {
                    if (!seen.count(next))
                        q.push(next);
                }
            }
        }
        return 0;
    }

    void dfs(TreeNode* root, TreeNode* p) {
        if (!root) return;
        g[root].push_back(p);
        g[p].push_back(root);
        dfs(root->left, root);
        dfs(root->right, root);
    }
};

Go

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func findClosestLeaf(root *TreeNode, k int) int {
	g := make(map[*TreeNode][]*TreeNode)
	var dfs func(root, p *TreeNode)
	dfs = func(root, p *TreeNode) {
		if root == nil {
			return
		}
		g[root] = append(g[root], p)
		g[p] = append(g[p], root)
		dfs(root.Left, root)
		dfs(root.Right, root)
	}
	dfs(root, nil)
	var q []*TreeNode
	for t, _ := range g {
		if t != nil && t.Val == k {
			q = append(q, t)
			break
		}
	}
	seen := make(map[*TreeNode]bool)
	for len(q) > 0 {
		node := q[0]
		q = q[1:]
		seen[node] = true
		if node != nil {
			if node.Left == nil && node.Right == nil {
				return node.Val
			}
			for _, next := range g[node] {
				if !seen[next] {
					q = append(q, next)
				}
			}
		}
	}
	return 0
}

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