学步园

Given a binary tree containing digits from 0-9 only, each root-to-leaf path could represent a number.

An example is the root-to-leaf path 1->2->3 which represents the number
123.

Find the total sum of all root-to-leaf numbers.

For example,

    1
   / \
  2   3

The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.

Return the sum = 12 + 13 = 25.

思路：

对二叉树进行递归搜索，同时记录迄今为止得到的值，当碰到叶结点的时候，就将这个值加在已知的和上。

题解：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int Sum;
    
    void TraverseProc(const TreeNode* node, int value)
    {
        const int VAL = value * 10 + node->val;
        
        if (node->left == nullptr && node->right == nullptr)
            Sum += VAL; // at a leaf
        else
        {
            if (node->left != nullptr)
                TraverseProc(node->left, VAL);
            if (node->right != nullptr) 
                TraverseProc(node->right, VAL);
        }
    }

    int sumNumbers(TreeNode *root) {
        Sum = 0;
        if (root != nullptr)
            TraverseProc(root, 0);
        return Sum;
    }
};