2698. Find the Punishment Number of an Integer

DFS

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class Solution {
    public int punishmentNumber(int n) {
        int result = 0;

        for (int i = 1; i <= n; i++) {
            if (match(i)) {
                result += i * i;
            }
        }

        return result;
    }

    private boolean match(int num) {
        int product = num * num;
        String productStr = String.valueOf(product);

        // 分割成多个，和为 num
        return dfs(productStr, 0, 0, num);
    }

    private boolean dfs(String productStr, int startIndex, int subSum, int targetSum) {
        if (subSum > targetSum) {
            return false;
        }

        if (startIndex == productStr.length()) {
            return subSum == targetSum;
        }

        int num = 0;
        for (int endIndex = startIndex; endIndex < productStr.length(); endIndex++) {
            num = num * 10 + (productStr.charAt(endIndex) - '0');
            if (dfs(productStr, endIndex + 1, subSum + num, targetSum)) {
                return true;
            }
        }

        return false;
    }
}

References

2698. Find the Punishment Number of an Integer