牛客题霸 [ 最长递增子序列] C++题解/答案

题目描述

给定数组arr，设长度为n，输出arr的最长递增子序列。（如果有多个答案，请输出其中字典序最小的）

题意：

直接暴力会超时
应该用二分+贪心

题解：

class Solution {
   
public:
    /** * retrun the longest increasing subsequence * @param arr int整型vector the array * @return int整型vector */
    vector<int> LIS(vector<int>& arr) {
   
        // write code here
         int n = arr.size();
        if (n < 2) return arr;
        int ans = 0;
        vector<int> ret(n);
        vector<int> st;
        for (int i = 0; i < n; i++){
   
            if (st.size() == 0 || arr[i] > st.back()){
   
                st.push_back(arr[i]);
                ret[i] = ++ans;
            }
            else{
   
                int l = 0, h = ans-1;
                while (l < h){
   
                    int m = (l+h)/2;
                    if (st[m] < arr[i]) l = m+1;
                    else h = m;
                }
                st[h] = arr[i];
                ret[i] = h+1;
            }
        }
        vector<int> res(ans);
        for (int i = n-1; i >= 0; i--){
   
            if (ret[i] == ans){
   
                res[--ans] = arr[i];
            }
        }
        return res;
    }
};