[LeetCode] Sum root to leaf numbers

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.

class Solution {public:    int sumNumbers(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int sum=0;        sumPath(root,0,sum);        return sum;    }    void sumPath(TreeNode* root, int prevSum, int& sum)    {        if(root==NULL)        {            return;        }        prevSum=prevSum*10+root->val;                if(root->left==NULL && root->right==NULL)            sum+=prevSum;                sumPath(root->left,prevSum,sum);        sumPath(root->right,prevSum,sum);    }};

树的递归两种策略：

1. 结果在root产生，可以遵循traversal order, 一般函数递归返回结果

2. 结果在leaf节点产生，并难以遵循traversal order, 并且产生结果需要父节点的数据，可以设置全局变量或者变量引用，在递归中变量得到更新，最终返回这个变量(如这题)