Minimum Depth of Binary Tree

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.

/**
 * 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) {
        vector<int> depth;
        if(root==NULL) return 0;
        calc(root, depth, 0);
        int min = depth[0];
        int len = depth.size();
        for(int i=1;i<len;i++){
            if(depth[i]<min)
                min = depth[i];
        }
        return min;
    }
    
    void calc(TreeNode *root, vector<int> &depth, int crt){
        if(root->left==NULL && root->right==NULL){
            depth.push_back(crt+1);
            return;
        }
        if(root->left)
            calc(root->left, depth, crt+1);
        if(root->right)
            calc(root->right, depth, crt+1);
    }
    
};