111. Minimum Depth of Binary Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int minDepth(TreeNode* root) {
        if(root == NULL)
        {
            return 0;
        }

        // if(root -> left == NULL)
        // {
        //     return 1 + minDepth(root -> right);
        // }
        
        // if(root -> right == NULL)
        // {
        //     return 1 + minDepth(root -> left);
        // }


        // int left = minDepth(root -> left);
        // int right = minDepth(root -> right);

        // return min(left, right) + 1;



        /* second approach using queue */

        queue<TreeNode*> q;
        q.push(root);

        int cnt = 0;

        while(!q.empty())
        {
            cnt++;

            int size = q.size();

            for(int i=0; i<size; i++)
            {
                TreeNode* front = q.front();
                q.pop();

                if(front -> left)
                {
                    q.push(front -> left);
                }

                if(front -> right)
                {
                    q.push(front -> right);
                }

                //we find the first leaf node
                if(front -> left == NULL && front -> right == NULL)
                    return cnt;
            }
        }


        return -1;

    }
};