【算法二十九】 437. 路径总和 III

437. 路径总和 III

前缀和+递归：

class Solution {
    private int ans;
    public int pathSum(TreeNode root, int targetSum) {
        Map<Long,Integer> count = new HashMap<>();
        count.put(0L,1);
        Long pre = 0L;
        dfs(root,targetSum,pre,count);
        return ans;
    }

    private void dfs(TreeNode node,int targetSum,Long pre,Map<Long,Integer> count){
        if(node == null){
            return;
        }
        pre += node.val;
        ans += count.getOrDefault(pre-targetSum,0);
        count.merge(pre,1,Integer::sum);
        dfs(node.left,targetSum,pre,count);
        dfs(node.right,targetSum,pre,count);
        //恢复，因为左边树弄完还有右边树
        count.merge(pre,-1,Integer::sum);
    }
}

时间复杂度：O(N)

空间复杂度：O(N)