Minimum Depth of Binary Tree

https://leetcode.com/problems/minimum-depth-of-binary-tree/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int minDepth(TreeNode* root) {
        queue<TreeNode *> q1;
        if(root==NULL)
            return 0;
        q1.push(root);
        int depth=0;
        bool flag=0;
        while(!q1.empty())
        {
            depth++;
            queue<TreeNode *> q2;
            while(!q1.empty())
            {
                TreeNode * temp=q1.front();
                q1.pop();
                if(temp->left==NULL&&temp->right==NULL)
                {
                    flag=1;
                    break;
                }
                if(temp->left!=NULL)
                    q2.push(temp->left);
                if(temp->right!=NULL)
                    q2.push(temp->right);
            }
            q1=q2;
            if(flag==1)
                break;
        }
        return depth;
    }
};