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.

minDepth.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (root == null) return 0;
        
        PriorityQueue<ArrayList<TreeNode>> que = new PriorityQueue<ArrayList<TreeNode>>();

        ArrayList<TreeNode> ary = new ArrayList<TreeNode>();
        ary.add(root);
        que.add(ary);
        int level = 1;
        
        while(!que.isEmpty()) {
            ArrayList<TreeNode> parents = que.poll();
            ArrayList<TreeNode> tmp = new ArrayList<TreeNode>();
            for (TreeNode parent: parents) {
                if (parent.left == null && parent.right == null) {
                    return level;
               }                

                if (parent.left != null) {
                    tmp.add(parent.left);
                }
                
                if (parent.right != null) {
                    tmp.add(parent.right);
                }
            }
            
            if (!tmp.isEmpty()) {
                que.add(tmp);
                level++;
            }
        }
        
        return 0;
    }
}