Unique Length-3 Palindromic Subsequences.js

// Runtime: 183 ms (Top 100.00%) | Memory: 52.8 MB (Top 44.44%)
var countPalindromicSubsequence = function(s) {
    const charToIndices = {};
    for (let i = 0; i < s.length; i++) {
        const char = s[i];
        if (charToIndices[char]) {
            charToIndices[char].push(i);
        } else {
            charToIndices[char] = [i];
        }
    }

    let count = 0;
    for (const currChar in charToIndices) {
        if (charToIndices[currChar].length < 2) continue;
        const currCharIndices = charToIndices[currChar];
        const firstIndex = currCharIndices[0];
        const lastIndex = currCharIndices[currCharIndices.length - 1];

        for (const char in charToIndices) {
            const indices = charToIndices[char];
            let lo = 0;
            let hi = indices.length;
            while (lo < hi) {
                const mid = (lo + hi) >> 1;
                if (indices[mid] <= firstIndex) {
                    lo = mid + 1;
                } else {
                    hi = mid;
                }
            }
            if (indices[lo] && indices[lo] < lastIndex) {
                count++;
            }
        }
    }
    return count;
};