【leetcode】113. Path Sum II

最新推荐文章于 2025-12-16 18:10:48 发布

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本文探讨了在给定的二叉树中寻找所有从根节点到叶子节点的路径，这些路径上的节点值之和等于指定的目标和。通过递归算法，我们详细介绍了如何遍历树结构并收集符合条件的路径，同时提供了清晰的代码实现。

题目：
Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum.

Note: A leaf is a node with no children.

思路：
遍历所有叶子节点，判断路径之和是否为sum。用一维数组t存放临时答案，用二维数组ans存放最终答案。

代码实现：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void f(TreeNode* root, int sum, vector<int> &t, vector<vector<int>> &ans){
        
        if (root->left == nullptr && root->right == nullptr){
            if (root->val == sum){
                t.push_back(root->val);
                ans.push_back(t);
                t.pop_back();
            }
        }else if(root->left == nullptr){
            t.push_back(root->val);
            f(root->right, sum-root->val, t, ans);
            t.pop_back();
        }else if(root->right == nullptr){
            t.push_back(root->val);
            f(root->left, sum-root->val, t, ans);
            t.pop_back();
        }else{
            t.push_back(root->val);
            f(root->right, sum-root->val, t, ans);
            f(root->left, sum-root->val, t, ans);
            t.pop_back();
        }
        
    }
    
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        
        vector<vector<int>> ans;
        if (root == nullptr){
            return ans;
        }
        vector<int> t;
        f(root, sum, t, ans);
        return ans;
    }
};

discuss:

// 常规思路
class Solution {
public:
    
    vector<vector<int>> pathSum(TreeNode* root, int sum) {
        vector<vector<int>> paths;
        vector<int> path;
        findPaths(root, sum, path, paths);
        return paths;
    }
    
    void findPaths(TreeNode *node, int sum, vector<int> &path, vector<vector<int>> &paths){
        if (!node) return ;
        path.push_back(node->val);
        if (!node->left && !node->right && sum == node->val) paths.push_back(path);
        findPaths(node->left, sum-node->val, path, paths);
        findPaths(node->right, sum-node->val, path, paths);
        path.pop_back();
    }
    
};