We want to find the shortest window such that the characters outside the window do not exceed n/4 for any character 'Q', 'W', 'E', 'R'.

1. Count the initial frequencies of 'Q', 'W', 'E', 'R' in s.
2. Target frequency target = n // 4.
3. If all frequencies <= target, return 0.
4. Use a sliding window [l, r]:
   • "Remove" s[r] from the counts of characters outside.
   • While all counts outside are <= target:
     • Update minimum length r - l + 1.
     • "Add" s[l] back to the counts outside and increment l.

• Time Complexity: O(N).
• Space Complexity: O(1).

from collections import Counter

def balanced_string(s):
    n = len(s)
    count = Counter(s)
    target = n // 4
    
    # Check if already balanced
    if all(count[c] <= target for c in "QWER"):
        return 0
        
    res = n
    l = 0
    for r in range(n):
        count[s[r]] -= 1
        while l < n and all(count[c] <= target for c in "QWER"):
            res = min(res, r - l + 1)
            count[s[l]] += 1
            l += 1
            
    return res