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Java-based LeetCode algorithm problem solutions, regularly updated

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package g1901_2000.s1927_sum_game;

// #Medium #Math #Greedy #Game_Theory #2022_05_14_Time_13_ms_(34.41%)_Space_43.1_MB_(80.65%)

/**
 * 1927 - Sum Game\.
 *
 * Medium
 *
 * Alice and Bob take turns playing a game, with **Alice starting first**.
 *
 * You are given a string `num` of **even length** consisting of digits and `'?'` characters. On each turn, a player will do the following if there is still at least one `'?'` in `num`:
 *
 * 1.  Choose an index `i` where `num[i] == '?'`.
 * 2.  Replace `num[i]` with any digit between `'0'` and `'9'`.
 *
 * The game ends when there are no more `'?'` characters in `num`.
 *
 * For Bob to win, the sum of the digits in the first half of `num` must be **equal** to the sum of the digits in the second half. For Alice to win, the sums must **not be equal**.
 *
 * *   For example, if the game ended with `num = "243801"`, then Bob wins because `2+4+3 = 8+0+1`. If the game ended with `num = "243803"`, then Alice wins because `2+4+3 != 8+0+3`.
 *
 * Assuming Alice and Bob play **optimally** , return `true` _if Alice will win and_ `false` _if Bob will win_.
 *
 * **Example 1:**
 *
 * **Input:** num = "5023"
 *
 * **Output:** false
 *
 * **Explanation:** There are no moves to be made. The sum of the first half is equal to the sum of the second half: 5 + 0 = 2 + 3.
 *
 * **Example 2:**
 *
 * **Input:** num = "25??"
 *
 * **Output:** true
 *
 * **Explanation:** Alice can replace one of the '?'s with '9' and it will be impossible for Bob to make the sums equal.
 *
 * **Example 3:**
 *
 * **Input:** num = "?3295???"
 *
 * **Output:** false
 *
 * **Explanation:** It can be proven that Bob will always win. One possible outcome is: 
 *
 * - Alice replaces the first '?' with '9'. num = "93295???". 
 *
 * - Bob replaces one of the '?' in the right half with '9'. num = "932959??". 
 *
 * - Alice replaces one of the '?' in the right half with '2'. num = "9329592?".
 *
 * - Bob replaces the last '?' in the right half with '7'. num = "93295927". 
 *   
 * Bob wins because 9 + 3 + 2 + 9 = 5 + 9 + 2 + 7.
 *
 * **Constraints:**
 *
 * *   2 <= num.length <= 10⁵
 * *   `num.length` is **even**.
 * *   `num` consists of only digits and `'?'`.
**/
public class Solution {
    public boolean sumGame(String num) {
        int count = 0;
        int diff = 0;
        int l = num.length();
        for (int i = 0; i < num.length(); i++) {
            if (num.charAt(i) == '?') {
                count += i < l / 2 ? 1 : -1;
            } else {
                if (i < l / 2) {
                    diff += num.charAt(i) - '0';
                } else {
                    diff -= num.charAt(i) - '0';
                }
            }
        }
        return diff * 2 != -9 * count;
    }
}