Minimum Depth of Binary Tree

描述

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.

分析

递归版

// Minimum Depth of Binary Tree
// 递归版,时间复杂度O(n),空间复杂度O(logn)
class Solution {
public:
    int minDepth(const 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 != NULL),
                minDepth(root->right, root->left != NULL));
    }
};

迭代版

// Minimum Depth of Binary Tree
// 迭代版,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) // 深度控制,剪枝
                s.push(make_pair(node->left, depth + 1));

            if (node->right && result > depth) // 深度控制,剪枝
                s.push(make_pair(node->right, depth + 1));
        }

        return result;
    }
};

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