437. Path Sum III

1. Question

Given the root of a binary tree and an integer targetSum, return the number of paths where the sum of the values along the path equals targetSum.

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

2. Examples

Example 1:

Input: root = [10,5,-3,3,2,null,11,3,-2,null,1], targetSum = 8
Output: 3
Explanation: The paths that sum to 8 are shown.

Example 2:

Input: root = [5,4,8,11,null,13,4,7,2,null,null,5,1], targetSum = 22
Output: 3

3. Constraints

• The number of nodes in the tree is in the range [0, 1000].
• -10⁹ <= Node.val <= 10⁹
• -1000 <= targetSum <= 1000

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/path-sum-iii 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

这道题需要两个递归，第一个递归是以某个节点开始每个都当作单独的树。第二个是在第一个的基础上进行dfs。

class Solution {
  public int pathSum(TreeNode root, int targetSum) {
    if (root == null) {
      return 0;
    }
    return getCount(root, targetSum) + pathSum(root.left, targetSum) + pathSum(root.right, targetSum);
  }

  private int getCount(TreeNode root, int sum) {
    int res = 0;
    if (root == null) {
      return res;
    }
    if (root.val == sum) {
      res++;
    }
    return res += getCount(root.left, sum - root.val) + getCount(root.right, sum - root.val);
  }
}