LeetCode: Minimum Depth of Binary Tree

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.

public class Solution {
    public int minDepth(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root==null) return 0;
        
        if(root.left==null && root.right==null) return 1;
        
        if(root.left==null){
            return 1+minDepth(root.right);
        }else if(root.right==null){
            return 1+minDepth(root.left);
        }else
            return 1+Math.min(minDepth(root.left), minDepth(root.right));        
    }
}

public class Solution {
    public int minDepth(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
       if(root==null) return 0;
       
       ArrayList<TreeNode> last =new ArrayList<TreeNode>();
       last.add(root);
       int count=1;
       while(!last.isEmpty()){           
        ArrayList<TreeNode> curr = new ArrayList<TreeNode>();
        for(TreeNode n:last){
           if(n.left==null && n.right==null) return count;
           if(n.left!=null) curr.add(n.left);
           if(n.right!=null) curr.add(n.right);
        }
        count++;
        last=new ArrayList<TreeNode>(curr);
       }
       return count;
    }
}