L437. Path Sum III

Problem:

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

Solution:

• res is like a counter

int res = 0;
public int pathSum(TreeNode root, int sum) {        
    if(root == null) return 0;
    helper(root,sum)
    pathSum(root.left, sum);
    pathSum(root.right,sum);
    return res;
}
private void helper(TreeNode root, int sum){
    if(root == null) return;
    if(sum - root.val == 0) res += 1;
    helper(root.left, sum - root.val);
    helper(root.right, sum - root.val);
}