2609. Find the Longest Balanced Substring of a Binary String 

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class Solution {
    public int findTheLongestBalancedSubstring(String s) {
        int maxLength = 0;

        for (int i = 0; i < s.length() - 1; i++) {
            // 中心扩散
            maxLength = Math.max(maxLength, tryExpand(s, i, i + 1));
        }

        return maxLength;
    }

    private int tryExpand(String s, int i, int j) {
        while (i >= 0 && j < s.length() && s.charAt(i) == '0' && s.charAt(j) == '1') {
            i--;
            j++;
        }
        return j - i - 1;        
    }
}
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class Solution {
    public int findTheLongestBalancedSubstring(String s) {
        int maxLength = 0;

        int[] counts = new int[2];
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '0') {
                if (i == 0 || s.charAt(i - 1) == '1') {
                    // 重置计数
                    counts[0] = 1;
                    counts[1] = 0;
                } else {
                    counts[0]++;
                }
            } else {
                // c == '1'
                counts[1]++;
                maxLength = Math.max(maxLength, 2 * Math.min(counts[0], counts[1]));
            }
        }

        return maxLength;
    }
}

References

2609. Find the Longest Balanced Substring of a Binary String