66. Plus One

题目：

Given a non-negative number represented as an array of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

链接：  http://leetcode.com/problems/plus-one/

题解：

Time Complexity - O(n)， Space Complexity - O(n)。

public class Solution {
    public int[] plusOne(int[] digits) {
        int carry = 1;
        for(int i = digits.length - 1; i >= 0; i --){
            int curValue = digits[i];
            digits[i] = (curValue + carry) % 10;
            carry = (curValue + carry) >= 10 ? 1 : 0;
        }
        
        if(carry == 1){
            int[] result = new int[digits.length + 1];
            result[0] = 1;
            for(int i = 1; i < result.length; i ++){
                result[i] = digits[i - 1];
            }
            return result;
        } else 
            return digits;
    }
}

Update:

public class Solution {
    public int[] plusOne(int[] digits) {
        if(digits == null || digits.length == 0)
            return digits;
        int carry = 1;
        
        for(int i = digits.length - 1; i >=0; i--) {
            int cur = digits[i];
            digits[i] = (cur + carry) % 10;
            carry = (cur + carry) >= 10 ? 1 : 0;
        }
        
        if(carry == 1) {
            int[] newDigits = new int[digits.length + 1];
            newDigits[0] = 1;
            
            for(int i = 1; i < newDigits.length; i++)
                newDigits[i] = digits[i - 1];
                
            return newDigits;
        }
        
        return digits;
    }
}

二刷：

注意carry最后为1的情况

Java:

Time Complexity - O(n)， Space Complexity - O(n)。

public class Solution {
    public int[] plusOne(int[] digits) {
        if (digits == null || digits.length == 0) {
            return digits;
        }
        int carry = 1, cur = 0;
        for (int i = digits.length - 1; i >= 0; i--) {
            cur = (digits[i] + carry) % 10;
            carry = (digits[i] + carry >= 10) ? 1 : 0;
            digits[i] = cur;
        }
        if (carry == 1) {
            int[] newDigits = new int[digits.length + 1];
            newDigits[0] = 1;
            for (int i = 1; i < newDigits.length; i++) {
                newDigits[i] = digits[i - 1];
            }
            return newDigits;
        } 
        return digits;
    }
}

三刷:

Java:

public class Solution {
    public int[] plusOne(int[] digits) {
        if (digits == null) {
            return digits;
        }
        int carry = 1;
        int len = digits.length;
        for (int i = len - 1; i >= 0; i--) {
            digits[i] += carry;
            carry = (digits[i] >= 10) ? 1 : 0;
            digits[i] %= 10;
        }
        if (carry == 1) {
            int[] res = new int[len + 1];
            res[0] = 1;
            for (int i = 1; i <= len; i++) {
                res[i] = digits[i - 1];
            }
            return res;
        }
        return digits;
    }
}

从luke处学到的方法，非常好用。

public class Solution {
    public int[] plusOne(int[] digits) {
        if (digits == null) {
            return digits;
        }
        int i = digits.length - 1;
        while (i >= 0) {
            if (digits[i] == 9) {
                digits[i--] = 0;
            } else {
                digits[i] += 1;
                return digits;
            }
        } 
        int[] res = new int[digits.length + 1];
        res[0] = 1;
        return res;
    }
}