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.

DFS

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

BFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        if (root != null) {
            Queue<TreeNode> queue = new LinkedList<>();
            queue.offer(root);
            int i = 0;
            while (queue.peek() != null) {
                i++;
                int k = queue.size();
                for (int j = 0; j < k; j++) {
                    TreeNode cur = queue.peek();
                    if (cur.left != null) queue.offer(cur.left);
                    if (cur.right != null) queue.offer(cur.right);
                    queue.poll();
                    if (cur.left == null && cur.right == null) return i;
                }
            }
        }
        return 0;
    }
}