x91. Decode Ways

Java clean DP solution with explanation - undefined - LeetCodeLeetCode

time: O(n)

space: O(n)

/**
 * @param {string} s
 * @return {number}
 */
var numDecodings = function(s) {
    if (s[0] == 0) return 0;
    if (s.length === 1) return 1;

    const arr = new Array(s.length).fill(0);
    arr[0] = 1;
    for (let i = 1; i < arr.length; i++) {
        const curr = s[i];
        const prev = s[i - 1];

        // 1
        if (curr != 0) {
            arr[i] += arr[i - 1];
        }

        // 2 
        if (prev == 1 || (prev == 2 && curr <= '6')) {
            arr[i] = i == 1 ? arr[i] + 1 : arr[i] + arr[i - 2];
        }
    }

    return arr[s.length - 1];
};