The minimum depth of a binary tree binary tree

https://www.bilibili.com/video/av46402848/

The first problem-solving approach

/ ** 
 * A for binary Tree Node Definition. 
 * Public class the TreeNode { 
 * int Val; 
 * the TreeNode left; 
 * the TreeNode right; 
 * the TreeNode (int X) {X = Val;} 
 *} 
 * / 
class Solution { 
    public int minDepth (the TreeNode the root) { 
        IF (the root == null) 
	    return 0; 
	IF (root.left == null) 
		return minDepth (root.right) + 1'd; //! ! ! ! ! Key left subtree is null see right subtree 
	IF (root.right == null) 
		return minDepth (root.left) +1; //! ! ! ! ! The key right subtree is null see left subtree 
	 int left = minDepth (root.left) +1; 
	 int right = minDepth (root.right) +1; 
	 return Math.min (left, right); //! ! ! ! ! The key 
    } 
}

The second linked list traversal to achieve levels 

class Solution {
	public int minDepth(TreeNode root) {
		int res = 0;
		if (root == null)
			return res;
		LinkedList<TreeNode> ll = new LinkedList<TreeNode>();
		ll.add(root);
		while (!ll.isEmpty()) {
			int size = ll.size();
			res++;                                                 // 要用两个while 的原因主要是增加层作用
			while (size > 0) {
				TreeNode temp = ll.poll();
				if (temp.left == null && temp.right == null)
					return res;
				if (temp.right != null)
					ll.add(temp.right);
				if (temp.left != null)
					ll.add(temp.left);
				size--;
			}
		}
		return res;
	}
}