problem：
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.
thinking：
（1）思路比较清晰：DFS遍历所有根节点到叶节点的路径，采用string类型保存较方便，最后再将string类型转换为int类型
（2）string类型表示的int数有可能溢出，需要额外注意，这道题的验证程序没有涉及。
code：
class Solution {
vector<string> str;
public:
int sumNumbers(TreeNode *root) {
if(root==NULL)
return 0;
string s;
dfs(root,s);
int ret=0;
for(int i=0;i<str.size();i++)
ret+=string_to_int(str[i]);
return ret;
}
void dfs(TreeNode *node, string s)
{
s.push_back('0'+node->val);
if(node->left==NULL && node->right==NULL)
{
str.push_back(s);
return;
}
if(node->left!=NULL)
dfs(node->left,s);
if(node->right!=NULL)
dfs(node->right,s);
}
int string_to_int(string s)
{
int n=s.size();
if(n==0)
return 0;
int num=0;
int a=1;
for(int i=n-1;i>=0;i--)
{
num+=(s.at(i)-'0')*a;
a*=10;
}
return num;
}
};
leetcode || 129、Sum Root to Leaf Numbers
原文：http://blog.csdn.net/hustyangju/article/details/45332613