LeetCode 738

LeetCode 738

class Solution {
    public int monotoneIncreasingDigits(int n) {
        StringBuilder sb = new StringBuilder(String.valueOf(n));
        int index = sb.length();
        if(checkUp(sb.toString(), index)){
            return Integer.parseInt(sb.toString());
        }
        while (index > 1){
            sb.replace(index-1, index, "9");
            char c = sb.charAt(index - 2);
            if(c == '0'){
                index--;
                continue;
            }
            sb.replace(index-2, index-1, String.valueOf(Character.digit(c, 10)-1));
            if(checkUp(sb.toString(), index-1)){
                return Integer.parseInt(sb.toString());
            }
            index--;
        }
        return Integer.parseInt(sb.toString());
    }

    public boolean checkUp(String s, int index){
        for(int i = 1; i < index; i++){
            if(s.charAt(i) < s.charAt(i-1)){
                return false;
            }
        }
        return true;
    }
}