题目链接：点击这里

类似于二叉树的前序遍历：

/**
 * 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 visit(TreeNode* r, int want, int now)
    {
        if(r==NULL) return false;
        if(r->left==NULL&&r->right==NULL)   //是叶子
            return now + r->val == want;
        else    //不是叶子
            return visit(r->left, want, now + r->val) || visit(r->right, want, now + r->val);
    }
    */
    bool visit(TreeNode* r, int want, int now)
    {
        if(r==NULL) return false;
        now += r->val;
        if(r->left==NULL&&r->right==NULL)   //是叶子
            return now == want;
        else
            return visit(r->left, want, now) || visit(r->right, want, now);
    }

    bool hasPathSum(TreeNode* root, int sum) {
        if(root==NULL)  return false;
        return visit(root, sum, 0);
    }
};