129. Sum Root to Leaf Numbers
题目
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.
解析
- 先序遍历的思想(根左右)+数字求和(每一层都比上层和*10+当前根节点的值)
- 可以从上到下累加，直到叶子节点，然后累加！
class Solution_129 {
public:
int dfs(TreeNode* root, int sum)
{
if (!root)
{
return;
}
sum = sum * 10 + root->val;
if (root->left==NULL&&root->right==NULL)
{
return sum;
}
return dfs(root->left, sum) + dfs(root->right, sum);
}
int sumNumbers_ref(TreeNode* root)
{
if (root == NULL)
{
return 0;
}
int sum = 0;
return dfs(root, sum); //从根节点开始
}
int sumNumbers(TreeNode *root) {
if (!root)
{
return 0;
}
stack<TreeNode*> sta;
sta.push(root);
int ret = 0;
TreeNode* top = 0;
while (!sta.empty())
{
top = sta.top();
sta.pop();
if (!top->left && !top->right)
{
ret += top->val;
}
if (top->left)
{
top->left->val += 10 * top->val; //但是这样改变了节点的值，可以将TreeNode* 和累积和(一个变量)组成一个pair，分开处理
sta.push(top->left);
}
if (top->right)
{
top->right->val += 10 * top->val;
sta.push(top->right);
}
}
return ret;
}
};
题目来源
