2731. Movement of Robots

Simulation(TLE)

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class Solution {
    private static final int MOD = 1000000007;

    public int sumDistance(int[] nums, String s, int d) {
        int[] directions = new int[nums.length];
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == 'L') {
                directions[i] = -1;
            } else {
                directions[i] = 1;
            }
        }

        for (int i = 0; i < d; i++) {
            Map<Integer, Set<Integer>> positionToRobotsMap = new HashMap<>(); // 注意每次新建 map, 不能全局使用一个 map, 因为每次全部机器人移动完才知道是否碰撞，不能使用之前的移动结果去进行比较

            for (int j = 0; j < nums.length; j++) {
                nums[j] += directions[j];

                Set<Integer> robots = positionToRobotsMap.computeIfAbsent(nums[j], key -> new HashSet<>());
                robots.add(j);
                if (robots.size() == 2) {
                    for (int robot : robots) {
                        directions[robot] *= -1;
                    }
                } else if (robots.size() > 2) {
                    directions[j] *= -1;
                }
            }
        }

        int sumDistance = 0;
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                sumDistance += (int) (Math.abs((long) nums[i] - nums[j]) % MOD);
                sumDistance %= MOD;
            }
        }

        return sumDistance;
    }
}

Math + Prefix Sum

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class Solution {
    private static final int MOD = 1000000007;

    public int sumDistance(int[] nums, String s, int d) {
        for (int i = 0; i < nums.length; i++) {
            nums[i] += d * (s.charAt(i) == 'L' ? -1 : 1);
        }

        Arrays.sort(nums);

        long distanceSum = 0;
        long prefixDistanceSum = 0;
        for (int i = 1; i < nums.length; i++) {
            prefixDistanceSum += (long) i * ((long) nums[i] - nums[i - 1]) % MOD; // 注意数字相减前的转型，以防止两个 int 整数相减越界
            distanceSum += prefixDistanceSum;
            distanceSum %= MOD;
        }

        return (int) distanceSum;
    }
}

References

2731. Movement of Robots