Minimum Depth of Binary Tree

题目：

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.

题解： 

递归解法急速判断左右两边子树哪个depth最小，要注意如果有个节点只有一边孩子时，不能返回0，要返回另外一半边的depth。 

递归解法：

public class Solution {
    public int minDepth(TreeNode root) 
    {
        int depth=0;
        depth=depthHelper(root,depth);
        return depth;
    }
    
    public int depthHelper(TreeNode root, int depth)
    {
        if(root==null)
        return 0;
        
        depth+=1;
        if(root.left!=null&&root.right!=null) 
        {
           return Math.min(depthHelper(root.left,depth),depthHelper(root.right,depth));
        }
        else if (root.left==null&&root.right!=null)
        {
            return depthHelper(root.right,depth);
        }
        else if(root.left!=null&&root.right==null)
        {
            return depthHelper(root.left,depth);
        }
        else return depth;

    }
}

解法二：

public class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return getMin(root);
    }

    public int getMin(TreeNode root){
        if (root == null) {
            return Integer.MAX_VALUE;
        }

        if (root.left == null && root.right == null) {
            return 1;
        }

        return Math.min(getMin(root.left), getMin(root.right)) + 1;
    }
}