111. 二叉树的最小深度

https://www.bilibili.com/video/av46402848/
考虑的方面要谨慎

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root==null)
             return 0;
         if(root.left==null)
            return minDepth(root.right)+1;   //！！！！！关键 左子树为null 就看 右子树
        if(root.right==null)
            return minDepth(root.left)+1;    //！！！！！关键 右子树为null 就看 左子树
         int left = minDepth(root.left) +1;
         int right = minDepth(root.right) +1;
         return Math.min(left,right);        //！！！！！关键
    }
}

用链表实现 层次遍历

class Solution {
    public int minDepth(TreeNode root) {
        int res = 0;
        if (root == null)
            return res;
        LinkedList ll = new LinkedList();
        ll.add(root);
        while (!ll.isEmpty()) {
            int size = ll.size();
            res++; // 要用两个while 的原因主要是增加层作用
            while (size > 0) {
                TreeNode temp = ll.poll();
                if (temp.left == null && temp.right == null)
                    return res;
                if (temp.right != null)
                    ll.add(temp.right);
                if (temp.left != null)
                    ll.add(temp.left);
                size--;
            }
        }
        return res;

    }
}