Breadth First Search

/*
1) Queues - nodes in each level
2) Level counter - level of tree
3) Size counter - number of nodes in each level
*/

// Sample program to find the minimum depth of a tree 
public int minDepth(TreeNode root) {
        if (root == null)
            return 0;
        Queue<TreeNode> queue = new LinkedList();
        int level = 1;                                              // Root has level 1
        queue.add(root);                                            // Add root to queue
        while (!queue.isEmpty()) {                                  // Loop until reaches last level
            int size = queue.size();  
            while (size > 0) {                                      // Loop through each nodes for one level
                TreeNode node = queue.poll();                       // Expand for each node in each level
                if (node.left == null && node.right == null)
                    return level;
                if (node.left != null)                              // Left subtree of each node 
                    queue.add(node.left);
                if (node.right != null)                             // Right subtree of each node
                    queue.add(node.right);
                size--;
            }
            level++;                                                // Moving to next level
        }
        return level;
    }