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.

思路

罗列清楚几种情况即可：

• root为null，返回0
• root的左右子节点为null，返回1
• 左右子节点都不为null，返回其中较小值
• 左右子节点存在一个，返回其depth

c++11

/**
 * 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) return 0;
        else if (!root->left and !root->right) return 1;
        else if (root->left and root->right) return std::min(minDepth(root->left), minDepth(root->right)) + 1;
        else return (root->left ? minDepth(root->left) : minDepth(root->right)) + 1;
    }
};