437. Path Sum III

You are given a binary tree in which each node contains an integer value.

Find the number of paths that sum to a given value.

The path does not need to start or end at the root or a leaf, but it must go downwards (traveling only from parent nodes to child nodes).

The tree has no more than 1,000 nodes and the values are in the range -1,000,000 to 1,000,000.

Example:

``````root = [10,5,-3,3,2,null,11,3,-2,null,1], sum = 8

10
/  \
5   -3
/ \    \
3   2   11
/ \   \
3  -2   1

Return 3. The paths that sum to 8 are:

1.  5 -> 3
2.  5 -> 2 -> 1
3. -3 -> 11
``````

``````class Solution {
public:
int pathSum(TreeNode* root, int sum) {
int res = 0;
vector<TreeNode*> out;
helper(root, sum, 0, out, res);
return res;
}
void helper(TreeNode* node, int sum, int curSum, vector<TreeNode*>& out, int& res) {
if (!node) return;
curSum += node->val;
out.push_back(node);
if (curSum == sum) ++res;
int t = curSum;
for (int i = 0; i < out.size() - 1; ++i) {
t -= out[i]->val;
if (t == sum) ++res;
}
helper(node->left, sum, curSum, out, res);
helper(node->right, sum, curSum, out, res);
out.pop_back();
}
};
``````

``````    1
/
2
/
1
``````

``````class Solution {
public:
int pathSum(TreeNode* root, int sum) {
unordered_map<int, int> m;
m[0] = 1;
return helper(root, sum, 0, m);
}
int helper(TreeNode* node, int sum, int curSum, unordered_map<int, int>& m) {
if (!node) return 0;
curSum += node->val;
int res = m[curSum - sum];
++m[curSum];
res += helper(node->left, sum, curSum, m) + helper(node->right, sum, curSum, m);
--m[curSum];
return res;
}
};
``````

``````class Solution {
public:
int pathSum(TreeNode* root, int sum) {
if (!root) return 0;
return sumUp(root, 0, sum) + pathSum(root->left, sum) + pathSum(root->right, sum);
}
int sumUp(TreeNode* node, int pre, int& sum) {
if (!node) return 0;
int cur = pre + node->val;
return (cur == sum) + sumUp(node->left, cur, sum) + sumUp(node->right, cur, sum);
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/437

Binary Tree Maximum Path Sum

Path Sum II

Path Sum

Minimum Path Sum

Path Sum IV

https://leetcode.com/problems/path-sum-iii/

https://leetcode.com/problems/path-sum-iii/discuss/91878/17-ms-O(n)-java-Prefix-sum-method

LeetCode All in One 题目讲解汇总(持续更新中…)

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