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.

tag: binary tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int ans = Integer.MAX_VALUE;
    public int minDepth(TreeNode root) {
        if (root == null) return 0;

        dfs(root, 1);

        return ans;
    }

    private void dfs(TreeNode root, int depth){
        if (root.left == null && root.right == null){
            ans = Math.min(ans, depth);
        }

        if (root.left != null) dfs(root.left, 1 + depth);
        if (root.right != null) dfs(root.right, 1 + depth);
    }
}