Question

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:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its minimum depth = 2.

Thinking:

  • Method 1:递归
    • 这道题有一种特殊情况，当输入为[1, 2],我们返回2. 
         			1
   		/
   	2
    • 我们需要判断左右结点是否为null。
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root == null) return 0;
        if(root.left == null)
            return Math.min(Integer.MAX_VALUE, minDepth(root.right)) + 1;
        else if(root.right == null)
            return Math.min(minDepth(root.left), Integer.MAX_VALUE) + 1;
        else
            return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
    }
}

二刷

还是在一刷的问题上卡了一下。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root == null) return 0;
        if(root.left == null && root.right != null)
            return Math.min(Integer.MAX_VALUE, minDepth(root.right)) + 1;
        else if(root.left != null && root.right == null)
            return Math.min(Integer.MAX_VALUE, minDepth(root.left)) + 1;
        else
            return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
    }
}