常用算法列举

Posted 2020-12-20 jason0529

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1. 深度优先遍历

使用场景：常见于各种组合，树遍历，前序遍历、中序遍历、后续遍历

private void dfs(int[] arr, int target,int begin, Deque<Integer> path, Set<List<Integer>> res) {
     if ( path.size()== target) {
        res.add(new ArrayList<>(path));
        return;
    }else if(begin>arr.length){
        return;
    }
    for (int i = begin; i < arr.length; i++) {
        path.add(arr[i]);
        dfs(arr,target,i+1,path,res);
        path.removeLast();
    }
}

2. 广度优先遍历

常见于树遍历，树层级，节点next指针等场景

public List<List<Integer>> levelOrderBottom(TreeNode root) {
 List<List<Integer>> levelOrder = new LinkedList<List<Integer>>();
        if (root == null) {
            return levelOrder;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<Integer> level = new ArrayList<Integer>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                level.add(node.val);
                TreeNode left = node.left, right = node.right;
                if (left != null) {
                    queue.offer(left);
                }
                if (right != null) {
                    queue.offer(right);
                }
            }
            levelOrder.add(0, level);
        }
        return levelOrder;


    }