leetcode解题: Path Sum (112)

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Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.

For example:
Given the below binary tree and sum = 22,
5
/ \
4 8
/ / \
11 13 4
/ \ \
7 2 1
return true, as there exist a root-to-leaf path 5->4->11->2 which sum is 22.

解法1： DFS

很直白的一个DFS的题目，用递归的方式很简洁的完成。基本思路是对每一个子节点，都可以看成是一个新的问题，只是问题变成了要求的sum是sum-root->val。
直到leaf的时候判断当前要求的sum和自己是否一致（或者是更新了的sum是否为0）
C++

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/**
 * 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:
    bool hasPathSum(TreeNode* root, int sum) {
        if (!root) {
            return false;
        }
        
        int cur = sum - root->val;
        if (!root->left && !root->right) {
            return cur == 0;
        }
        
        return hasPathSum(root->left, cur) || hasPathSum(root->right, cur);
    }
};

Java

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