23. Sum Root to Leaf Numbers

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.

思想： 分三种情况： 1. 当前结点为空，返回0.  2. 叶子结点， 返回当前值.  3. 父结点，返回左右两个 path 值的和。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 int dfs(TreeNode *root, int sum) {
    if(root == 0) return 0;
    sum = sum * 10 + root->val;
    if(root->left || root->right)
        return dfs(root->left, sum) + dfs(root->right, sum);
    return sum;
 }
class Solution {
public:
    int sumNumbers(TreeNode *root) {
        return dfs(root, 0);
    }
};