leetcode 111二叉树的最小深度

使用深度优先搜索：时间复杂度O(n),空间复杂度O(logn)

/**
 * Definition for a binary tree node.
 * 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==NULL) return ;
        //当左右子树有一个为空时，深度按不为空的算；
        if(root->left==NULL) return +minDepth(root->right);
        if(root->right==NULL) return +minDepth(root->left);
        //当左右子树都不为空时，深度按照小的算；
        return +min(minDepth(root->right),minDepth(root->left));
    }
};

精简版：

/**
 * Definition for a binary tree node.
 * 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==NULL) return ;
        int leftDepth=minDepth(root->left);
        int rightDepth=minDepth(root->right);
        return (leftDepth== || rightDepth==)? leftDepth+rightDepth+:+min(leftDepth,rightDepth);
    }
};