leetcode213周赛

Posted 2020-12-21 BeicC

5556. 可以到达的最远建筑

刚开始写得时候感觉像贪心，但没分析好不敢写，自己写了个记忆化搜索，后来看讲解把贪心法给补上

记忆化搜索

const int MAXN = 1e5+50;
class Solution {
public:
    int dp[MAXN];
    int dfs(int now, vector<int>& heights, int bricks, int ladders){
        if (dp[now] != -1) return dp[now];
        int n = heights.size();
        if (now == n-1) return n-1;
        while (now+1 < n && heights[now] > heights[now+1]) now++;
        if (now == n-1) return n-1;
        int a = now,b = now;
        if (ladders > 0) a = dfs(now+1, heights, bricks, ladders-1);
        if (bricks >= heights[now+1]-heights[now]) b = dfs(now+1, heights, bricks-(heights[now+1]-heights[now]), ladders);
        dp[now] = max(a,b);
        return dp[now];
    }
    int furthestBuilding(vector<int>& heights, int bricks, int ladders) {
        int n = heights.size();
        for (int i = 0; i <= n; i++) dp[i] = -1;
        return dfs(0,heights, bricks, ladders);
    }
};

贪心:

/*贪心法就是把梯子先存着，将梯子用在高度差比较大的地方*/
class Solution {
public:
    priority_queue<int> que;
    int furthestBuilding(vector<int>& heights, int bricks, int ladders) {
        while (!que.empty()) que.pop();
        int ans, sum = 0;
        for (int i = 0; i < heights.size(); i++){
            ans = i;
            if (i == heights.size()-1) break;
            int a = heights[i], b = heights[i+1];
            if (a >= b) {
                //pass
            }else{
                que.push(a-b);
                if (que.size() > ladders){
                    sum -= que.top();que.pop();
                }
                if (sum > bricks) break;
            }
        }
        return ans;
    }
};

5600. 第 K 条最小指令

在答案的每个位置只有两种选择，不是H就是V，按字典序排序后，H肯定在前面而且一字排开，范围是1-C[h+v-1] [h-1]，如果超过这个范围，那肯定就是V打头.(通过这题再一次复习了一下组合数及其写法..)

const int MAXN = 32;
class Solution {
public:
    int C[MAXN][MAXN];
    void init(){
        C[0][0] = 1;
        for (int i = 1; i < MAXN; i++){
            C[i][0] = 1;
            for (int j = 1; j <= i; j++){
                C[i][j] = C[i-1][j] + C[i-1][j-1];
            }
        }
    }
    string kthSmallestPath(vector<int>& d, int k) {
        init();
        int h = d[1], v = d[0], n = h+v;
        string ans;
        for (int i = 0; i < n; i++){
            if (h > 0 && k <= C[h+v-1][h-1]){
                h--;
                ans += "H";
            }else{
                if (h > 0) k -= C[h+v-1][h-1];
                else k -= C[h+v-1][0];
                v--;
                ans += "V";
            }
        }
        return ans;
    }
};

总结：组合数及其写法???♂?

    void init(){
        C[0][0] = 1;
        for (int i = 1; i < MAXN; i++){
            C[i][0] = 1;
            for (int j = 1; j <= i; j++){
                C[i][j] = C[i-1][j] + C[i-1][j-1];
            }
        }
    }