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.

Solution: 1. Recursion. Pay attention to cases when the non-leaf node has only one child.
2. Iteration + Queue.

/**
 * 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) return 0;
        if(!root->left && !root->right) return 1;
        if(!root->left) {
            return 1 + minDepth(root->right);
        }
        if(!root->right) {
            return 1 + minDepth(root->left);
        }
        return 1 + min(minDepth(root->left), minDepth(root->right));
    }
};