LintCode-480: Binary Tree Paths

原创已于 2022-10-17 01:22:04 修改 · 185 阅读

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于 2018-06-12 13:56:06 首次发布

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本文介绍了使用递归和分治策略实现的二叉树所有根到叶子节点路径的查找算法，并提供了详细的C++代码实现。

我用的递归，代码如下：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: the root of the binary tree
     * @return: all root-to-leaf paths
     */
    vector<string> binaryTreePaths(TreeNode * root) {
        vector<string> result;
        if (!root) return result;
        string path;
        findPath(root, path, result);
        return result;
    }

private:
    void findPath(TreeNode *root, string & path, vector<string> & result) {
        if (root && !root->left && !root->right) { 
            result.push_back(path+to_string(root->val));    
        } else {
            string tempStr=path + to_string(root->val) + "->";
            if (root->left) 
                findPath(root->left, tempStr, result);
            if (root->right)
                findPath(root->right, tempStr, result);
         }
    }
};

注意:

C++里面可以用to_string(int)来把一个int变成string。类似的方法还有springf(), stringstream, etc.

二刷:

class Solution {
public:
    /**
     * @param root: the root of the binary tree
     * @return: all root-to-leaf paths
     *          we will sort your return value in output
     */
    vector<string> binaryTreePaths(TreeNode *root) {
        vector<string> res;
        string str;
        helper(root, res, str);
        return res;
    }
private:
    void helper(TreeNode *root, vector<string> &res, string str) {
        if (!root) return;
        str = str + to_string(root->val);
        if (!root->left && !root->right) {
            res.push_back(str);
            return;
        }
        str = str + "->";
        helper(root->left, res, str);
        helper(root->right, res, str);
        return;
    }
};

还有一个方法是用分治，九章参考答案如下，我感觉写的很好。下次再学习。

public List<String> binaryTreePaths(TreeNode root) {
        List<String> paths = new ArrayList<>();
        if (root == null) {
            return paths;
        }
        
        List<String> leftPaths = binaryTreePaths(root.left);
        List<String> rightPaths = binaryTreePaths(root.right);
        for (String path : leftPaths) {
            paths.add(root.val + "->" + path);
        }
        for (String path : rightPaths) {
            paths.add(root.val + "->" + path);
        }
        
        // root is a leaf
        if (paths.size() == 0) {
            paths.add("" + root.val);
        }
        
        return paths;
    }