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

最短路径的节点个数。注意左子树或右子树为空的情况。

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;
        if (!root->left) return minDepth(root->right) + 1;
        if (!root->right) return minDepth(root->left) + 1;
        return std::min(minDepth(root->left), minDepth(root->right)) + 1;
    }
};

AC结果：

Runtime: 24 ms, faster than 20.72% of C++ online submissions for Minimum Depth of Binary Tree.
Memory Usage: 19.9 MB, less than 46.18% of C++ online submissions for Minimum Depth of Binary Tree.