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514. Freedom Trail Hard

In the video game Fallout 4, the quest "Road to Freedom" requires players to reach a metal dial called the "Freedom Trail Ring", and use the dial to spell a specific keyword in order to open the door.

Given a string ring, which represents the code engraved on the outer ring and another string key, which represents the keyword needs to be spelled. You need to find the minimum

 number of steps in order to spell all the characters in the keyword.

Initially, the first character of the ring is aligned at 12:00 direction. You need to spell all the characters in the string key one by one by rotating the ring clockwise or anticlockwise to make each character of the string key aligned at 12:00 direction and then by pressing the center button. 

At the stage of rotating the ring to spell the key character key[i]:
  1. You can rotate the ring clockwise or anticlockwise one place, which counts as 1 step. The final purpose of the rotation is to align one of the string ring's characters at the 12:00 direction, where this character must equal to the character key[i]
    .
  2. If the character key[i] has been aligned at the 12:00 direction, you need to press the center button to spell, which also counts as 1 step. After the pressing, you could begin to spell the next character in the key (next stage), otherwise, you've finished all the spelling.
思路:本題明顯使用動態規劃可以避免重複子問題的計算從而減少時間開銷。 需要求出來的是轉到每一個字元所需要的最少轉數,將這些最小轉數加起來就得到想要的結果。通過匹配的方法,從key中一一取出一個字元,在ring中匹配,得到轉到該字元的最小轉數。注意,除了第一個字元外,剩下的字元的轉數需要由上一個字元的轉數加上轉動次數決定。本題需要考慮的是當出現了順、逆時針轉動時都有相同轉數的匹配,這是應該選擇順時針還是逆時針,這裡需要將兩種結果與下一個匹配的結果相加進行比較來選擇。結尾的+key.length()是因為選中後需要按button,每次按button算一次步驟。
class Solution {
public:
    int findRotateSteps(string ring, string key) {
        int result[key.length()][ring.length()];
        int answer = INT_MAX;
        for (int i = 0; i < key.length(); i++) {
            for (int j = 0; j < ring.length(); j++) {
                result[i][j] = INT_MAX;
            }
        }
        
        for (int i = 0; i < key.length(); i++) {
             for (int j = 0; j < ring.length(); j++) {
                 if (key[i] == ring[j]) {
                     if (i == 0)
                        result[i][j] = min(abs(i - j), (int)ring.length() - abs(i - j));
                     if (i != 0)
                    for (int k = 0; k < ring.length(); k++) {
                        if (result[i - 1][k] != INT_MAX) {
                            result[i][j] = min(result[i][j], result[i - 1][k] + min(abs(k - j), (int)ring.length() - abs(k - j)));
                            if (i == key.length() - 1 && answer > result[i][j]) {
                                answer = result[i][j];
                            }
                        }
                    }
                 }
             }
        }
        return answer + key.length();
    }
};