题目：

题解：

class Solution {public int pathSum(TreeNode root, int targetSum) {Map<Long, Integer> prefix = new HashMap<Long, Integer>();prefix.put(0L, 1);return dfs(root, prefix, 0, targetSum);}public int dfs(TreeNode root, Map<Long, Integer> prefix, long curr, int targetSum) {if (root == null) {return 0;}int ret = 0;curr += root.val;ret = prefix.getOrDefault(curr - targetSum, 0);prefix.put(curr, prefix.getOrDefault(curr, 0) + 1);ret += dfs(root.left, prefix, curr, targetSum);ret += dfs(root.right, prefix, curr, targetSum);prefix.put(curr, prefix.getOrDefault(curr, 0) - 1);return ret;}
}