[GeeksForGeeks] Find Minimum Depth of a Binary Tree

Give a binary tree, find its minimum depth. 

The minimum depth is the number of nodes along the shortest path from root node down to the nearest leaf node.

Solution 1. Recursion 

public class MinDepthBt {
    public int getMinDepth(TreeNode root) {
        return helper(root, 0);
    }
    private int helper(TreeNode node, int depth) {
        if(node == null) {
            return depth;
        }
        return Math.min(helper(node.left, depth + 1), helper(node.right, depth + 1));
    }
}

Solution 2. Level order traversal, found the first leaf node and return its level.

This solution is better than the recursive solution because we don't need to traverse all nodes.

import java.util.LinkedList;
import java.util.Queue;

public class MinDepthBt {
    public int getMinDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        if(root.left == null || root.right == null) {
            return 1;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int currLevel = 1; int minLevel = 1;
        boolean minDepthFound = false;
        while(!minDepthFound && !queue.isEmpty()) {
            int size = queue.size();
            for(int i = 0; i < size; i++) {
                TreeNode curr = queue.poll();
                if(curr.left == null && curr.right == null) {
                    minDepthFound = true;
                    minLevel = currLevel;
                    break;
                }
                if(curr.left != null) {
                    queue.offer(curr.left);
                }
                if(curr.right != null) {
                    queue.offer(curr.right);
                }
            }
            currLevel++;
        }
        return minLevel;
    }
}