Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {

    public int pathSum(TreeNode root, int sum) {
        if (root == null) return 0;
        int res = 0;
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            res += solution(node, sum);
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
        }
        return res;
    }

    private int solution(TreeNode root, int sum) {
        if (root == null) return 0;
        int res = sum == root.val ? 1 :
        return solution(root.left, sum - root.val) + solution(root.right, sum - root.val) + res;
    }
}