216. 组合总和 iii

function combinationSum3(k: number, n: number): number[][] {
    let result = [];
    let path = [];
    let sum = 0;

    const backTracking = (k: number, startIndex: number) => {
        if (path.length === k && sum === n) {
            result.push([...path]);
            return;
        };

        for (let i = startIndex; i <= 9 - (k - path.length) + 1; i++) {
            path.push(i);
            sum += i;
            if (sum > n) {
                path.pop();
                sum -= i;
                return;
            }
            backTracking(k, i + 1);
            path.pop();
            sum -= i;
        }
    };

    backTracking(k, 1);
    return result;
};

17. 电话号码的字母组合

function letterCombinations(digits: string): string[] {
    let result = [];
    let path = [];
    const k = digits.length;
    const map = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"];

    if (k === 0) {
        return result;
    }

    if (k === 1) {
        return map[digits].split('');
    }

    const backtracking = (index) => {
        if (path.length === k) {
            result.push(path.join(''));
            return;
        }

        for(let value of map[digits[index]]) {
            path.push(value);
            backtracking(index + 1);
            path.pop();
        }
    };

    backtracking(0);
    return result;
};