111. Minimum depth of binary tree

Force link 111. Minimum depth of binary tree
Given a binary tree , Find out the minimum depth .

The minimum depth is the number of nodes on the shortest path from the root node to the nearest leaf node .

explain ： A leaf node is a node that has no children .

Example 1：

Input ：root = [3,9,20,null,null,15,7]
Output ：2
Example 2：

Input ：root = [2,null,3,null,4,null,5,null,6]
Output ：5

Tips ：
The number of nodes in the tree ranges from [0, 105] Inside
-1000 <= Node.val <= 1000

Their thinking

Sequence traversal , If there are leaf nodes in this layer , The current number of layers is the minimum depth .

Realization

class Solution {
    public int minDepth(TreeNode root) {
         // Sequence traversal   iteration 
        Queue<TreeNode> queue = new LinkedList<>();
        int deep = 0;
        if(root == null){
            return deep;
        }
        queue.offer(root);
        int flag = 0;
        while(!queue.isEmpty()){
              deep++;
            int len = queue.size();
            while(len-->0){
                TreeNode t  = queue.poll();
                if(t.left == null && t.right == null){
                    flag =1;
                    break;
                }

                if(t.left != null) queue.offer(t.left);
                if(t.right != null) queue.offer(t.right);
            }
            if(flag == 1) break;
        }
        return deep;
    }
}