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/**
 * 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
 */

public class Solution{
    public int minDepth(TreeNode root) {
        return _minDepth(root, 1);
    }
    public int _minDepth(TreeNode root, int depth){
        if(root == null)
            return depth - 1;
        int d_left = _minDepth(root.left, depth + 1);
        int d_right = _minDepth(root.right, depth + 1);
        if(d_left == depth)
            return d_right;
        if(d_right == depth)
            return d_left;
        return d_left < d_right ? d_left : d_right;
    }
}

/**
 * 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) {
        if(root == null) return 0;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        q.offer(null);
        int count = 1;
        while(!q.isEmpty()){
            TreeNode curr = q.poll();
            if(curr != null){
                if(curr.left == null && curr.right == null){
                    return count;
                }
                if(curr.left != null){
                    q.offer(curr.left);
                }
                if(curr.right != null){
                    q.offer(curr.right);
                }   
            }else{
                if(!q.isEmpty()){
                    count++;
                    q.offer(null);
                }
            }
        }
        return count;
    }
}