111. 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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
return its minimum depth = 2.

class Solution {
public:
    int minDepth(TreeNode* root) {
        if(!root) {
            return 0;
        }
        if(!root->left){
            return 1+minDepth(root->right);
        }
        if(!root->right){
            return 1+minDepth(root->left);
        }
        int l = minDepth(root->left);

        int r = minDepth(root->right);

        return l < r ? l + 1: r + 1;
    }
};