力扣labuladong一刷day7共3题

力扣labuladong一刷day7共3题

一、216. 组合总和 III

题目链接：https://leetcode.cn/problems/combination-sum-iii/
思路：还是组合只是既有n又有k。

class Solution {
   List<List<Integer>> arrayLists = new ArrayList<>();
    List<Integer> list = new ArrayList<>();
    int sum = 0;
    public List<List<Integer>> combinationSum3(int k, int n) {
        backTracking(k, n, 1);
        return arrayLists;
    }
    void backTracking(int k, int n, int index) {
        if (list.size() == k && sum == n) {
            arrayLists.add(new ArrayList<>(list));
            return;
        }
        if (list.size() > k) return;
        for (int i = index; i <= 9 && sum+i <= n; i++) {
            sum += i;
            list.add(i);
            backTracking(k, n, i+1);
            sum -= i;
            list.remove(list.size()-1);
        }
    }
}

二、111. 二叉树的最小深度

题目链接：https://leetcode.cn/problems/minimum-depth-of-binary-tree/
思路：采用层序遍历。

public int minDepth(TreeNode root) {
        if (root == null) return 0;
        Deque<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int deep = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            deep++;
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
                if (node.left == null && node.right == null) {
                    return deep;
                }
            }
        }
        return deep;
    }

三、752. 打开转盘锁

题目链接：https://leetcode.cn/problems/open-the-lock/
思路：相当于一个点每次周围有8个点可以走，bfs。

class Solution {
  public int openLock(String[] deadends, String target) {
        Set<String> visited = new HashSet<>();
        Set<String> set = new HashSet<>();
        for (String deadend : deadends) {
            set.add(deadend);
        }
        Deque<String> queue = new LinkedList<>();
        queue.add("0000");
        visited.add("0000");
        int keep = 0;
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                String cur = queue.poll();
                if (cur.equals(target)) return keep;
                if (set.contains(cur)) continue;
                for (int j = 0; j < 4; j++) {
                    String up = upOne(cur, j);
                    if (!visited.contains(up)) {
                        queue.add(up);
                        visited.add(up);
                    }
                    String down = downOne(cur, j);
                    if (!visited.contains(down)) {
                        queue.add(down);
                        visited.add(down);
                    }
                }
            }
            keep++;
        }
        return -1;
    }
    String upOne(String s, int i) {
        char[] chars = s.toCharArray();
        if (chars[i] == '9') {
            chars[i] = '0';
        }else {
            chars[i] += 1;
        }
        return new String(chars);
    }
    String downOne(String s, int i) {
        char[] chars = s.toCharArray();
        if (chars[i] == '0'){
            chars[i] = '9';
        }else {
            chars[i] -= 1;
        }
        return new String(chars);
    }

}