Number of Ways to Select Buildings — LeetCode 2222 Python Solution

The problem

You are given a 0-indexed binary string s which represents the types of buildings along a street where: s[i] = '0' denotes that the ith building is an office and s[i] = '1' denotes that the ith building is a restaurant. As a city official, you would like to select 3 buildings for random inspection.

Example

Input

s = "001101"

Output

6

Explanation

The following sets of indices selected are valid:

Python solution

class Solution:
    def numberOfWays(self, s: str) -> int:
        l = [0, 0]
        r = [s.count("0"), s.count("1")]
        ans = 0
        for x in map(int, s):
            r[x] -= 1
            ans += l[x ^ 1] * r[x ^ 1]
            l[x] += 1
        return ans

Complexity

Pattern: Prefix Sum

Precompute running totals once so any range query becomes a single subtraction. LeetCode 2222. Number of Ways to Select Buildings is filed here because LeetCode tags it Prefix Sum, which is the vocabulary this hub collects.

The prefix sum guide has the Python template for the pattern and the 157 LeetCode problems that use it.

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