[Leetcode]-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.

Hide Tags :Tree , Depth-first Search
题目：求取二叉树中最小深度，根节点定义为1深度。

思路：相同採取递归
递归终止条件：
　　　　　A：假设root为NULL。说明已经越界，返回深度0
推断节点走向：
　　　　　Ａ：假设root为树叶， if(root->left == NULL && root->right == NULL) 返回深度1
　　　　　Ｂ：假设节点无左节点，即if(root->left == NULL ) ，则运行递归return minDepth(root->right) + 1;
　　　　　Ｃ：假设节点无右节点，则return minDepth(root->left ) + 1;
递归主进程：
int l = minDepth(root->left) + 1 ;
int r = minDepth(root->right)+ 1 ;
最后在推断ｌ与ｒ的大小，返回小者return min(l,r);

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
#define min(a,b) (((a)<(b))?(a):(b))
int minDepth(struct TreeNode* root) {

    if(root == NULL)                                return 0;
    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;
    int l = minDepth(root->left) + 1 ;
    int r = minDepth(root->right)+ 1 ;
    return min(l,r); 

}