[LeetCode-111] Minimum Depth of Binary Tree （二叉树最小深度）

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.

本题就是求解二叉树的深度： 
递归解法： 
（1）如果二叉树为空，二叉树的深度为0 
（2）如果二叉树不为空，二叉树的深度 = min(左子树深度， 右子树深度) + 1

（3）需要注意的是斜二叉树情况，当节点的左子树深度为0或者节点的右子树深度为0.

采用DFS深度优先遍历，我们只要叶子节点。

/**
* Definition for a binary tree node.
*/
struct TreeNode {
int val;
struct TreeNode *left;
struct TreeNode *right;
};

int minDepth(TreeNode *root)
{
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (root == NULL)
return 0;

if (root->left == NULL && root->right == NULL)
return 1;

int leftDepth = minDepth(root->left);
int rightDepth = minDepth(root->right);

if (leftDepth == 0)
return rightDepth + 1;
else if (rightDepth == 0)
return leftDepth + 1;
else
return leftDepth < rightDepth ? (leftDepth +1 ) : (rightDepth +1);
}