Formatted question description: https://leetcode.ca/all/2192.html

# 2192. All Ancestors of a Node in a Directed Acyclic Graph (Medium)

You are given a positive integer `n`

representing the number of nodes of a **Directed Acyclic Graph** (DAG). The nodes are numbered from `0`

to `n - 1`

(**inclusive**).

You are also given a 2D integer array `edges`

, where `edges[i] = [from`

denotes that there is a _{i}, to_{i}]**unidirectional** edge from `from`

to _{i}`to`

in the graph._{i}

Return *a list* `answer`

*, where *`answer[i]`

* is the list of ancestors of the*

`i`^{th}

*node, sorted in*.

**ascending order**A node `u`

is an **ancestor** of another node `v`

if `u`

can reach `v`

via a set of edges.

**Example 1:**

Input:n = 8, edgeList = [[0,3],[0,4],[1,3],[2,4],[2,7],[3,5],[3,6],[3,7],[4,6]]Output:[[],[],[],[0,1],[0,2],[0,1,3],[0,1,2,3,4],[0,1,2,3]]Explanation:The above diagram represents the input graph. - Nodes 0, 1, and 2 do not have any ancestors. - Node 3 has two ancestors 0 and 1. - Node 4 has two ancestors 0 and 2. - Node 5 has three ancestors 0, 1, and 3. - Node 6 has five ancestors 0, 1, 2, 3, and 4. - Node 7 has four ancestors 0, 1, 2, and 3.

**Example 2:**

Input:n = 5, edgeList = [[0,1],[0,2],[0,3],[0,4],[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]Output:[[],[0],[0,1],[0,1,2],[0,1,2,3]]Explanation:The above diagram represents the input graph. - Node 0 does not have any ancestor. - Node 1 has one ancestor 0. - Node 2 has two ancestors 0 and 1. - Node 3 has three ancestors 0, 1, and 2. - Node 4 has four ancestors 0, 1, 2, and 3.

**Constraints:**

`1 <= n <= 1000`

`0 <= edges.length <= min(2000, n * (n - 1) / 2)`

`edges[i].length == 2`

`0 <= from`

_{i}, to_{i}<= n - 1`from`

_{i}!= to_{i}- There are no duplicate edges.
- The graph is
**directed**and**acyclic**.

**Related Topics**:

Depth-First Search, Breadth-First Search, Graph, Topological Sort

**Similar Questions**:

## Solution 1. DFS

```
// OJ: https://leetcode.com/problems/all-ancestors-of-a-node-in-a-directed-acyclic-graph/
// Time: O(N^2)
// Space: O(N^2)
class Solution {
public:
vector<vector<int>> getAncestors(int n, vector<vector<int>>& E) {
vector<vector<int>> G(n), ans(n);
bool connect[1001][1001] = {};
for (auto &e : E) G[e[0]].push_back(e[1]);
function<void(int, int)> dfs = [&](int u, int s) {
if (connect[s][u]) return; // If `s` has been added as a source node of `u` already, skip
connect[s][u] = true;
for (int v : G[u]) dfs(v, s);
};
for (int i = 0; i < n; ++i) dfs(i, i);
for (int i = 0; i < n; ++i) {
for (int j = 0; j < n; ++j) {
if (i != j && connect[i][j]) ans[j].push_back(i);
}
}
return ans;
}
};
```

## Solution 2.

```
// OJ: https://leetcode.com/problems/all-ancestors-of-a-node-in-a-directed-acyclic-graph/
// Time: O(N^2)
// Space: O(E)
class Solution {
public:
vector<vector<int>> getAncestors(int n, vector<vector<int>>& E) {
vector<vector<int>> G(n), ans(n);
for (auto &e : E) G[e[0]].push_back(e[1]);
function<void(int, int)> dfs = [&](int u, int s) {
for (int v : G[u]) {
if (ans[v].size() && ans[v].back() == s) continue;
ans[v].push_back(s);
dfs(v, s);
}
};
for (int i = 0; i < n; ++i) dfs(i, i);
return ans;
}
};
```