Minimum Depth of Binary Tree

Problem:

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.

Solution:
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int minDepth(TreeNode *root) {
        if (root == NULL)
            return 0;
        else if (root -> left == NULL && root -> right == NULL)
            return 1;
        else if (root -> left == NULL)
            return minDepth(root -> right) + 1;
        else if (root -> right == NULL)
            return minDepth(root -> left) + 1;
        else
            return min(minDepth(root -> left), minDepth(root -> right)) + 1;
    }
};

======= BFS approach ==== (Use of pair<node *, depth>)

class Solution {

public:

    int minDepth(TreeNode *root) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        queue< pair<TreeNode*,int> > q;

        int i=0;

        if (!root){return 0;}

        q.push(make_pair(root,1));

        while(!q.empty()){

            pair<TreeNode*,int> cur = q.front();

            q.pop();

            if (!cur.first->left && !cur.first->right){

                return cur.second;

            }

            if(cur.first->left){

                q.push(make_pair(cur.first->left,cur.second+1));

            }

            if(cur.first->right){

                q.push(make_pair(cur.first->right,cur.second+1));

            }

        }

    }

};