[Leetcode]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.

递归：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) {
return minDepth(root, false);
}
private:
static int minDepth(const TreeNode* root, bool hasbrother) {
if (!root)
return hasbrother? INT_MAX : 0;
return 1 + min(minDepth(root->left, root->right != nullptr), minDepth(root->right, root->left != nullptr));
}
};迭代：时间复杂度O(n)，空间复杂度O(logn)。

class Solution {
public:
int minDepth(TreeNode* root) {
if (root == nullptr)
return 0;

int result = INT_MAX;
stack<pair<TreeNode*, int>> s;
s.push(make_pair(root, 1));
while (!s.empty()) {
auto node = s.top().first;
auto depth = s.top().second;
s.pop();
if (node->left == nullptr && node->right == nullptr)
result = min(result, depth);
if (node->left && result > depth)
s.push(make_pair(node->left, depth + 1));
if (node->right && result > depth)
s.push(make_pair(node->right, depth + 1));
}
return result;
}
};