技術標籤：LeetCode演算法與資料結構leetcodetreebfsdfs

Topic

• Tree
• Breadth-first Search
• Depth-first Search

Description

https://leetcode.com/problems/minimum-depth-of-binary-tree/

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example 1:

    3
   / \
  9  20
     / \
   15   7

Input: root = [3,9,20,null,null,15,7]
Output: 2

Example 2:

2
 \
  3
   \
    4
     \
      5
       \
        6

Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5

Constraints:

• The number of nodes in the tree is in the range [0, 10⁵].
• -1000 <= Node.val <= 1000

Analysis

方法一：DFS

方法二：BFS

Submission

import java.util.LinkedList;
import java.util.Queue;

import com.lun.util.BinaryTree.TreeNode;

public class MinimumDepthOfBinaryTree {
	
	//方法一：DFS
	public int minDepth1(TreeNode root) {
		if (root == null)
			return 0;
		int left = minDepth1
 
(root.left), right = minDepth1(root.right);
		return (left == 0 || right == 0) ? Math.max(left, right) + 1 : Math.min(left, right) + 1;//不是簡單的返回Math.min(left, right) + 1，當左右子樹其中一個高度為0，則選其中最大高度者（也就是另一子樹高度）+ 1。這有點反直覺。
	}

	//方法二：BFS
	public int minDepth2(TreeNode root) {
		if (root == null) return 0;
		int depth = 1;
		Queue<TreeNode> q = new LinkedList<TreeNode>();
		q.offer(root);
		while (!q.isEmpty()) {
			int size = q.size();
			// for each level
			for (int i = 0; i < size; i++) {
				TreeNode node = q.poll();
				if (node.left == null && node.right == null) {
					return depth;
				}
				if (node.left != null) {
					q.offer(node.left);
				}
				if (node.right != null) {
					q.offer(node.right);
				}
			}
			depth++;
		}
		return depth;
	}
}

Test

import static org.junit.Assert.*;
import org.junit.Test;

import com.lun.util.BinaryTree.TreeNode;

public class MinimumDepthOfBinaryTreeTest {

	@Test
	public void test1() {
		MinimumDepthOfBinaryTree obj = new MinimumDepthOfBinaryTree();

		TreeNode root = new TreeNode(3);
		
		root.left = new TreeNode(9);
		root.right = new TreeNode(20);
		
		root.right.left = new TreeNode(15);
		root.right.right = new TreeNode(7);
		
		assertEquals(2, obj.minDepth1(root));
		assertEquals(2, obj.minDepth2(root));
	}
	
	@Test
	public void test2() {
		MinimumDepthOfBinaryTree obj = new MinimumDepthOfBinaryTree();
		
		TreeNode root = new TreeNode(2);
		root.right = new TreeNode(3);
		root.right.right = new TreeNode(4);
		root.right.right.right = new TreeNode(5);
		root.right.right.right.right = new TreeNode(6);

		assertEquals(5, obj.minDepth1(root));
		assertEquals(5, obj.minDepth2(root));
	}
}