LeetCode 2657. Find the Prefix Common Array of Two Arrays
You are given two 0-indexed integer permutations A and B of length n.
A prefix common array of A and B is an array C such that C[i] is equal to the count of numbers that are present at or before the index i in both A and B.
Return the prefix common array of A and B.
A sequence of n integers is called a permutation if it contains all integers from 1 to n exactly once.
Example 1:
Input: A = [1,3,2,4], B = [3,1,2,4]
Output: [0,2,3,4]
Explanation:
At i = 0: no number is common, so C[0] = 0.
At i = 1: 1 and 3 are common in A and B, so C[1] = 2.
At i = 2: 1, 2, and 3 are common in A and B, so C[2] = 3.
At i = 3: 1, 2, 3, and 4 are common in A and B, so C[3] = 4.
Example 2:
Input: A = [2,3,1], B = [3,1,2]
Output: [0,1,3]
Explanation:
At i = 0: no number is common, so C[0] = 0.
At i = 1: only 3 is common in A and B, so C[1] = 1.
At i = 2: 1, 2, and 3 are common in A and B, so C[2] = 3.
Constraints:
1 <= A.length == B.length == n <= 501 <= A[i], B[i] <= nIt is guaranteed that A and B are both a permutation of n integers.
判断前i个位置有相同的数字的个数；
这里面用一个集合去存储两个数组前i个数，当下一个出线新的数字的时候，如果存在就++；
当然如果2个相等，直接++即可；
所有可能性都想到了，就返回即可；
一开始是想用的笨的方法；以后还是多想想快速的解决方案了。。。
Runtime: 5 ms, faster than 77.64% of Java online submissions for Find the Prefix Common Array of Two Arrays.
Memory Usage: 43.7 MB, less than 15.92% of Java online submissions for Find the Prefix Common Array of Two Arrays.
