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1182. Shortest Distance to Target Color πŸ”’

Description

You are given an array colors, in which there are three colors: 1, 2 and 3.

You are also given some queries. Each query consists of two integers i and c, return the shortest distance between the given index i and the target color c. If there is no solution return -1.

 

Example 1:

Input: colors = [1,1,2,1,3,2,2,3,3], queries = [[1,3],[2,2],[6,1]]
Output: [3,0,3]
Explanation: 
The nearest 3 from index 1 is at index 4 (3 steps away).
The nearest 2 from index 2 is at index 2 itself (0 steps away).
The nearest 1 from index 6 is at index 3 (3 steps away).

Example 2:

Input: colors = [1,2], queries = [[0,3]]
Output: [-1]
Explanation: There is no 3 in the array.

 

Constraints:

  • 1 <= colors.length <= 5*10^4
  • 1 <= colors[i] <= 3
  • 1 <= queries.length <= 5*10^4
  • queries[i].length == 2
  • 0 <= queries[i][0] < colors.length
  • 1 <= queries[i][1] <= 3

Solutions

Solution 1: Preprocessing

We can preprocess the distance from each position to the nearest color $1$, $2$, $3$ on the left, and the distance from each position to the nearest color $1$, $2$, $3$ on the right, and record them in the arrays $left$ and $right$. Initially, $left[0][0] = left[0][1] = left[0][2] = -\infty$, and $right[n][0] = right[n][1] = right[n][2] = \infty$, where $n$ is the length of the array colors.

Then for each query $(i, c)$, the minimum distance is $d = \min(i - left[i + 1][c - 1], right[i][c - 1] - i)$. If $d > n$, there is no solution, and the answer to this query is $-1$.

The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the length of the array colors.

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class Solution:
    def shortestDistanceColor(
        self, colors: List[int], queries: List[List[int]]
    ) -> List[int]:
        n = len(colors)
        right = [[inf] * 3 for _ in range(n + 1)]
        for i in range(n - 1, -1, -1):
            for j in range(3):
                right[i][j] = right[i + 1][j]
            right[i][colors[i] - 1] = i
        left = [[-inf] * 3 for _ in range(n + 1)]
        for i, c in enumerate(colors, 1):
            for j in range(3):
                left[i][j] = left[i - 1][j]
            left[i][c - 1] = i - 1
        ans = []
        for i, c in queries:
            d = min(i - left[i + 1][c - 1], right[i][c - 1] - i)
            ans.append(-1 if d > n else d)
        return ans

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class Solution {
    public List<Integer> shortestDistanceColor(int[] colors, int[][] queries) {
        int n = colors.length;
        final int inf = 1 << 30;
        int[][] right = new int[n + 1][3];
        Arrays.fill(right[n], inf);
        for (int i = n - 1; i >= 0; --i) {
            for (int j = 0; j < 3; ++j) {
                right[i][j] = right[i + 1][j];
            }
            right[i][colors[i] - 1] = i;
        }
        int[][] left = new int[n + 1][3];
        Arrays.fill(left[0], -inf);
        for (int i = 1; i <= n; ++i) {
            for (int j = 0; j < 3; ++j) {
                left[i][j] = left[i - 1][j];
            }
            left[i][colors[i - 1] - 1] = i - 1;
        }
        List<Integer> ans = new ArrayList<>();
        for (int[] q : queries) {
            int i = q[0], c = q[1] - 1;
            int d = Math.min(i - left[i + 1][c], right[i][c] - i);
            ans.add(d > n ? -1 : d);
        }
        return ans;
    }
}

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class Solution {
public:
    vector<int> shortestDistanceColor(vector<int>& colors, vector<vector<int>>& queries) {
        int n = colors.size();
        int right[n + 1][3];
        const int inf = 1 << 30;
        fill(right[n], right[n] + 3, inf);
        for (int