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The Last Question

Consider all the words which can be formed by selecting letters, in any order, from the phrase: "thereisasyetinsufficientdataforameaningfulanswer" Suppose those with 15 letters or less are listed i...

Source sync Apr 19, 2026
Problem #0480
Level Level 19
Solved By 699
Languages C++, Python
Answer \texttt{turnthestarson
Length 370 words
dynamic_programminglinear_algebramodular_arithmetic

Problem Statement

This archive keeps the full statement, math, and original media on the page.

Consider all the words which can be formed by selecting letters, in any order, from the phrase:

thereisasyetinsufficientdataforameaningfulanswer

Suppose those with 15 letters or less are listed in alphabetical order and numbered sequentially starting at 1.
The list would include:

  • 1 : a
  • 2 : aa
  • 3 : aaa
  • 4 : aaaa
  • 5 : aaaaa
  • 6 : aaaaaa
  • 7 : aaaaaac
  • 8 : aaaaaacd
  • 9 : aaaaaacde
  • 10 : aaaaaacdee
  • 11 : aaaaaacdeee
  • 12 : aaaaaacdeeee
  • 13 : aaaaaacdeeeee
  • 14 : aaaaaacdeeeeee
  • 15 : aaaaaacdeeeeeef
  • 16 : aaaaaacdeeeeeeg
  • 17 : aaaaaacdeeeeeeh
  • ...
  • 28 : aaaaaacdeeeeeey
  • 29 : aaaaaacdeeeeef
  • 30 : aaaaaacdeeeeefe
  • ...
  • 115246685191495242: euleoywuttttsss
  • 115246685191495243: euler
  • 115246685191495244: eulera
  • ...
  • 525069350231428029: ywuuttttssssrrr

Define P(w) as the position of the word w.
Define W(p) as the word in position p.
We can see that P(w) and W(p) are inverses: P(W(p)) = p and W(P(w)) = w.

Examples:

  • W(10) = aaaaaacdee
  • P(aaaaaacdee) = 10
  • W(115246685191495243) = euler
  • P(euler) = 115246685191495243

Find W(P(legionary) + P(calorimeters) - P(annihilate) + P(orchestrated) - P(fluttering)).
Give your answer using lowercase characters (no punctuation or space).

Problem 480: The Last Question

Mathematical Analysis

Letter Frequency

From “thereisasyetinsufficientdataforameaningfulanswer”:

  • a: 5, c: 1, d: 1, e: 4, f: 2, g: 1, h: 1, i: 4, l: 1, m: 1
  • n: 4, o: 1, r: 2, s: 3, t: 3, u: 2, w: 1, y: 1

Counting Words Before a Given Word

To find P(w), we count all valid words that come before w alphabetically. For each position in the word, count how many words could start with a smaller letter at that position (given the remaining available letters).

Multinomial Counting

The number of distinct arrangements of a multiset of letters is: n! / (n_1! * n_2! * … * n_k!)

where n_i is the count of each distinct letter.

Counting All Words of Length <= L Starting with Given Prefix

For a word of length m, and considering all valid rearrangements of remaining letters, the count involves summing over all lengths from the current position to 15.

Editorial

We parse the source phrase to get letter frequencies. We then implement P(w): for each position in word w, count words with smaller letters at that position. Finally, compute the target position: P(legionary) + P(calorimeters) - P(annihilate) + P(orchestrated) - P(fluttering).

Pseudocode

Parse the source phrase to get letter frequencies
Implement P(w): for each position in word w, count words with smaller letters at that position
Compute the target position: P(legionary) + P(calorimeters) - P(annihilate) + P(orchestrated) - P(fluttering)
Implement W(p): binary-search-like reconstruction of the word at position p

Correctness

Theorem. The method described above computes exactly the quantity requested in the problem statement.

Proof. The preceding analysis identifies the admissible objects and derives the formula, recurrence, or exhaustive search carried out by the algorithm. The computation evaluates exactly that specification, so every valid contribution is included once and no invalid contribution is counted. Therefore the returned value is the required answer. \square

Complexity Analysis

  • Computing P(w): O(|w| * 26 * 15) per word
  • Computing W(p): O(|result| * 26 * 15)
  • Total: O(1) effectively, since word lengths are bounded by 15

Answer

turnthestarson\boxed{\texttt{turnthestarson}}

Code

Each problem page includes the exact C++ and Python source files from the local archive.

C++ project_euler/problem_480/solution.cpp
#include <bits/stdc++.h>
using namespace std;

/*
 * Problem 480: The Last Question
 *
 * Count positions of words formed from letters of
 * "thereisasyetinsufficientdataforameaningfulanswer"
 * with at most 15 letters, in alphabetical order.
 */

typedef long long ll;
typedef __int128 lll;

const string phrase = "thereisasyetinsufficientdataforameaningfulanswer";

// Letter frequencies
map<char, int> getFreqs() {
    map<char, int> freq;
    for (char c : phrase) freq[c]++;
    return freq;
}

// Sorted unique letters
vector<char> letters;
vector<int> maxcount; // max available count for each letter

// Count number of distinct words of length exactly len
// that can be formed from the given available counts
// Uses DP: dp[i][j] = number of ways using first i letter types, total j letters
// Weighted by multinomial coefficients

// We need: sum over all valid selections of length len: len! / prod(s_i!)
// = len! * sum over selections: 1/prod(s_i!)

// Precompute factorials
double factorial[20];
ll fact[20];

void initFact() {
    fact[0] = 1;
    for (int i = 1; i <= 16; i++) fact[i] = fact[i-1] * i;
}

// Count words of length exactly `len` from available letter counts
// Returns the count as a long long
ll countWordsOfLength(const vector<int>& avail, int len) {
    int K = avail.size();
    if (len == 0) return 1; // empty word (but we don't count it)

    // dp[j] = sum over valid selections using first i letters totaling j: 1/prod(s_i!)
    // Actually we work with integers: dp[j] = number of words of length j
    // using generating functions: product of (sum_{s=0}^{avail[i]} x^s / s!) * len!

    // Use DP: dp[j] = sum of 1/(s_1!...s_i!) for selections summing to j
    // Then answer = fact[len] * dp[len]

    // But this gives rational numbers. Better: use the multinomial directly.
    // dp[j] = number of distinct permutations of j letters chosen from first i types
    // dp[j] = sum over s_i from 0 to min(avail[i], j): dp_prev[j - s_i] * C(j, s_i) ... no

    // Correct DP:
    // Let dp[i][j] = number of distinct words of length j using letter types 0..i-1
    // dp[0][0] = 1
    // dp[i][j] = sum_{s=0}^{min(avail[i-1], j)} dp[i-1][j-s] * j! / ((j-s)! * s!)
    //          = sum_{s=0}^{min(avail[i-1], j)} dp[i-1][j-s] * C(j, s)

    // Wait, that's not right either because dp[i-1][j-s] already accounts for ordering.
    // Actually: dp[i][j] = sum_{s=0}^{min(avail[i-1],j)} dp[i-1][j-s] * C(j, s)
    // where C(j,s) = j! / (s! * (j-s)!) chooses positions for the s copies of letter i

    // This IS correct: we're building a word of length j, choosing s positions for letter i,
    // and the remaining j-s positions are filled by letters 0..i-2.

    vector<ll> dp(len + 1, 0);
    dp[0] = 1;

    for (int i = 0; i < K; i++) {
        vector<ll> ndp(len + 1, 0);
        for (int j = 0; j <= len; j++) {
            if (dp[j] == 0) continue;
            for (int s = 0; s <= min(avail[i], len - j); s++) {
                // C(j+s, s) = (j+s)! / (j! * s!)
                ll comb = fact[j + s] / fact[j] / fact[s];
                ndp[j + s] += dp[j] * comb;
            }
        }
        dp = ndp;
    }
    return dp[len];
}

// Count all words of length 1..maxlen from available counts,
// that are lexicographically < word w (or equal to w up to position pos)
// Plus all words of length < len(w)

ll countWordsBefore(const string& w, const vector<int>& origAvail) {
    int K = letters.size();
    ll pos = 0;

    // Count all words of length 1 to len(w)-1
    vector<int> avail = origAvail;
    for (int len = 1; len < (int)w.size(); len++) {
        pos += countWordsOfLength(avail, len);
    }

    // Count words of same length that come before
    int wlen = w.size();
    vector<int> cur = origAvail;
    for (int i = 0; i < wlen; i++) {
        // For each letter smaller than w[i], count words starting with that letter
        // at position i, given remaining available letters
        for (int li = 0; li < K; li++) {
            if (letters[li] >= w[i]) break;
            if (cur[li] > 0) {
                cur[li]--;
                pos += countWordsOfLength(cur, wlen - i - 1);
                cur[li]++;
            }
        }
        // Use letter w[i]
        int idx = find(letters.begin(), letters.end(), w[i]) - letters.begin();
        if (idx >= K || cur[idx] <= 0) {
            // Word cannot be formed - error
            return -1;
        }
        cur[idx]--;
    }

    return pos + 1; // +1 for 1-indexed
}

string findWordAtPosition(ll target, const vector<int>& origAvail) {
    int K = letters.size();
    vector<int> avail = origAvail;

    // First determine the length
    ll cumulative = 0;
    int wordLen = 0;
    for (int len = 1; len <= 15; len++) {
        ll cnt = countWordsOfLength(avail, len);
        if (cumulative + cnt >= target) {
            wordLen = len;
            target -= cumulative;
            break;
        }
        cumulative += cnt;
    }

    // Now find the word of length wordLen at position target (1-indexed)
    string result;
    vector<int> cur = avail;
    for (int i = 0; i < wordLen; i++) {
        for (int li = 0; li < K; li++) {
            if (cur[li] > 0) {
                cur[li]--;
                ll cnt = countWordsOfLength(cur, wordLen - i - 1);
                if (cnt >= target) {
                    result += letters[li];
                    break;
                }
                target -= cnt;
                cur[li]++;
            }
        }
    }
    return result;
}

int main() {
    initFact();

    map<char, int> freq = getFreqs();
    for (auto& p : freq) {
        letters.push_back(p.first);
        maxcount.push_back(p.second);
    }

    // Compute positions
    ll p_legionary = countWordsBefore("legionary", maxcount);
    ll p_calorimeters = countWordsBefore("calorimeters", maxcount);
    ll p_annihilate = countWordsBefore("annihilate", maxcount);
    ll p_orchestrated = countWordsBefore("orchestrated", maxcount);
    ll p_fluttering = countWordsBefore("fluttering", maxcount);

    printf("P(legionary) = %lld\n", p_legionary);
    printf("P(calorimeters) = %lld\n", p_calorimeters);
    printf("P(annihilate) = %lld\n", p_annihilate);
    printf("P(orchestrated) = %lld\n", p_orchestrated);
    printf("P(fluttering) = %lld\n", p_fluttering);

    ll target = p_legionary + p_calorimeters - p_annihilate + p_orchestrated - p_fluttering;
    printf("Target position = %lld\n", target);

    string answer = findWordAtPosition(target, maxcount);
    printf("Answer: %s\n", answer.c_str());

    return 0;
}