# Java 6ms topological sort solution with explanation

• This problem is a classic graph topological sort problem. Each prerequisite has edges to the courses that require it.

We define in degree as the number of edges into a node in the graph. What we do is we remove the nodes that has in degree equals to 0, decrease the in degree of the nodes that require the current node, and repeat, until we've removed all the nodes (the successful case), or there's no node with in degree equals to 0 (the failed case).

So, we can do this using a priority queue, and make the in degree as the priority. Every time we poll a node from the queue, and decrease the priorities of the children of the node. If the polled node has in degree larger than 1, it means we failed. But since Java's priority queue doesn't support convenient decrease key operation, we have to remove one node and add it back, which causes bad performance.

Or we can use two pointers. We put the removed node in an array, and use a left pointer to iterate through the array and decrease the in degrees of the nodes than require the current node. And use a right pointer to add those nodes which have 0 in degree after the decreasing operation. Repeat this until all nodes are added.

``````public class Solution {
public int[] findOrder(int numCourses, int[][] prerequisites) {

int[] inDeg = new int[numCourses];
List<Integer>[] chl = new ArrayList[numCourses];

for (int i = 0; i < numCourses; i++) {
chl[i] = new ArrayList<Integer>();
}

int pre;
int cour;

for (int[] pair : prerequisites) {
pre = pair[1];
cour = pair[0];

inDeg[cour]++;
}

int[] res = new int[numCourses];
int k = 0;

for (int i = 0; i < numCourses; i++) {
if (inDeg[i] == 0) {
res[k++] = i;
}
}

if (k == 0) {
return new int[0];
}

int j = 0;
List<Integer> tmp;

while (k < numCourses) {
tmp = chl[res[j++]];

for (int id : tmp) {
if (--inDeg[id] == 0) {
res[k++] = id;
}
}

if (j == k) {
return new int[0];
}
}

return res;
}
}``````

• @magicyuli awesome solution without using Queue but with the same idea, thanks for sharing this.

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