DFS BFS

Key points:

• DFS-stack&recursion, BFS-queue

• If BFS works, never use DFS because of StackOverflow, important! like this one: surrounded-regions

• DFS:

  • A and B is connected?

  • Connected component. (Union Find is faster because no need to build graph)

  • Search path

  • Has cycle

  • two color

  • Backtracking, all results

• BFS:

  • shortest path

  • connected component

  • topological sorting

  • level order traversal

  • Topological sorting

• Need to mark visited element:

  • if you need to get value, use HashMap

  • if just determine contain or not, use HashSet or boolean[]

Templates

BFS in a tree

• Queue -> q.offer(root) -> while -> levelNum = q.size() -> for loop

• if you don't need layer, for loop is not required.

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Queue<TreeNode> q = new Queue<TreeNode>();

        if (root == null) return res;

        q.offer(root);
        while (!q.isEmpty()) {
            int levelNum = q.size();
            List<Integer> tmpList = new ArrayList<Integer>();
            for (int i = 0; i < levelNum; i++) {
                TreeNode curr = q.poll();
                tmpList.add(curr.val);
                if (curr.left != null) q.offer(curr.left);
                if (curr.right != null) q.offer(curr.right);
            }
            res.add(tmpList);
        }
        return res;
    }
}

Permutation & Combination