[CF777D] Cloud of Hashtags（贪心，二分）

题目链接：http://codeforces.com/contest/777/problem/D

题意：n个字符串，保留前缀。希望保留尽可能多的字符，使得字典序单增。

把n个字符串翻转过来，相当于字典序递减。比较相邻两个字符串的字典序大小，用二分将当前字符串截至恰好比上一个字符串字典序小，且最长即可。

#include <bits/stdc++.h>
using namespace std;

const int maxn = 500500;
int n;
string a[maxn], b[maxn];

signed main() {
    // freopen("in", "r", stdin);
    while(~scanf("%d", &n)) {
        for(int i = n; i >= 1; i--) cin >> a[i];
        b[1] = a[1];
        for(int i = 2; i <= n; i++) {
            if(a[i] <= b[i-1]) {
                b[i] = a[i];
                continue;
            }
            int lo = 0, hi = a[i].length();
            string ret;
            while(lo <= hi) {
                int mid = (lo + hi) >> 1;
                if(a[i].substr(0, mid) <= b[i-1]) {
                    ret = a[i].substr(0, mid);
                    lo = mid + 1;
                }
                else hi = mid - 1;
            }
            b[i] = ret;
        }
        for(int i = n; i >= 1; i--) cout << b[i] << endl;
    }
    return 0;
}