111. Minimum Depth of Binary Tree

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DFS

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class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        } else if (root.left == null) {
            return minDepth(root.right) + 1;
        } else if (root.right == null) {
            return minDepth(root.left) + 1;
        } else {
            return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
        }
    }
}

BFS

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class Solution {
    public int minDepth(TreeNode root) {
        int depth = 0;

        Queue<TreeNode> queue = new LinkedList<>();
        if (root != null) {
            queue.offer(root);
        }

        while (!queue.isEmpty()) {
            depth++;
            
            for (int i = queue.size(); i > 0; i--) {
                TreeNode node = queue.poll();
                if (node.left == null && node.right == null) {
                    return depth;
                }
                
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
        }

        return depth;
    }
}

References

111. Minimum Depth of Binary Tree