[leetcode] 129. sum root to leaf numbers 求根到叶节点数字之和
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.
note: a leaf is a node with no children.
example:
input: [1,2,3]
1
/ \
2 3
output: 25
explanation:
the root-to-leaf path
1->2
represents the number
12
.
the root-to-leaf path
1->3
represents the number
13
.
therefore, sum = 12 + 13 =
25
.
example 2:
input: [4,9,0,5,1]
4
/ \
9 0
/ \
5 1
output: 1026
explanation:
the root-to-leaf path
4->9->5
represents the number 495.
the root-to-leaf path
4->9->1
represents the number 491.
the root-to-leaf path
4->0
represents the number 40.
therefore, sum = 495 + 491 + 40 =
1026
.
这道求根到叶节点数字之和的题跟之前的求 path sum 很类似，都是利用dfs递归来解，这道题由于不是单纯的把各个节点的数字相加，而是每遇到一个新的子结点的数字，要把父结点的数字扩大10倍之后再相加。如果遍历到叶结点了，就将当前的累加结果sum返回。如果不是，则对其左右子结点分别调用递归函数，将两个结果相加返回即可，参见代码如下：
解法一：
class solution {
public:
int sumnumbers(treenode* root) {
return sumnumbersdfs(root, 0);
}
int sumnumbersdfs(treenode* root, int sum) {
if (!root) return 0;
sum = sum * 10 + root->val;
if (!root->left && !root->right) return sum;
return sumnumbersdfs(root->left, sum) + sumnumbersdfs(root->right, sum);
}
};
我们也可以采用迭代的写法，这里用的是先序遍历的迭代写法，使用栈来辅助遍历，首先将根结点压入栈，然后进行while循环，取出栈顶元素，如果是叶结点，那么将其值加入结果res。如果其右子结点存在，那么其结点值加上当前结点值的10倍，再将右子结点压入栈。同理，若左子结点存在，那么其结点值加上当前结点值的10倍，再将左子结点压入栈，是不是跟之前的 path sum 极其类似呢，参见代码如下：
解法二：
class solution {
public:
int sumnumbers(treenode* root) {
if (!root) return 0;
int res = 0;
stack<treenode*> st{{root}};
while (!st.empty()) {
treenode *t = st.top(); st.pop();
if (!t->left && !t->right) {
res += t->val;
}
if (t->right) {
t->right->val += t->val * 10;
st.push(t->right);
}
if (t->left) {
t->left->val += t->val * 10;
st.push(t->left);
}
}
return res;
}
};
github 同步地址：
类似题目：
path sum
binary tree maximum path sum
参考资料：
https://leetcode.com/problems/sum-root-to-leaf-numbers/discuss/41367/non-recursive-preorder-traverse-java-solution
https://leetcode.com/problems/sum-root-to-leaf-numbers/discuss/41452/iterative-c%2b%2b-solution-using-stack-(similar-to-postorder-traversal)
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