LeetCode - Algorithms - 7. Reverse Integer

Java

1

自己做出来的naive solution，直接把整数转换成字符串，然后逆转字符串

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class Solution {
    public int reverse(int x) {
    	int r = 0;
    	if (x>Integer.MAX_VALUE || x<Integer.MIN_VALUE) {
    		return r;
    	}
    	String s = Integer.toString(x);
    	if (s!=null && !s.isEmpty()) {
    		String ss = "";
    		char c = 0;
    		if (x<0)
    			s = s.substring(1);
    		for(int i=s.length()-1;i>=0;i--) {
    			c = s.charAt(i);
    			ss += c;
    		}
    		if (x<0)
    			ss = '-'+ss;
    		System.out.println("ss="+ss);
    		try {
    		    r = Integer.parseInt(ss);
    		} catch (NumberFormatException ex) {
    			r = 0;
    		}
    	}    		
        return r;        
    }
}

Submission Detail

  • 1032 / 1032 test cases passed.

2

做 9. Palindrome Number ，提示Coud you solve it without converting the integer to a string? 发现人家的方法更直接利落，温故而知新，重新实现下，结果被卡在 java int out of range 的问题里，1534236469 逆序数超过Integer.MAX_VALUE 返回 1056389759，题目里就提示了 Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows. 最后想到不如把逆序数变量定义为 long类型，再判断是否超过2147483647(0x7fffffff, 2^31-1)，再把long 安全地转换为int返回，java的long安全转换为int的小问题，google关键字 java long to int 参考了下 Safely casting long to int in Java 这样就ok了，在leetcode上运行通过。

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class Solution {
    public int reverse(int x) {
    	if (x>Integer.MAX_VALUE || x<Integer.MIN_VALUE) {
    		return 0;
    	}
    	if (x==0) return 0;
    	int r = 0;
    	int remainder = 0;
    	long reverseNumber = 0L;
    	for(int t = x>0?x:-x; t>0; t/=10) {
    		remainder = t%10;
    		reverseNumber *= 10;
    		reverseNumber += remainder;
    	}
    	if (reverseNumber>Integer.MAX_VALUE)
    		r = 0;
    	else
    		r = Long.valueOf(reverseNumber).intValue();
    	if (x<0)
    		r *= -1;    	
        return r;        
    }
}

Submission Detail

  • 1032 / 1032 test cases passed.
  • Runtime: 21 ms
  • Your runtime beats 97.62 % of java submissions.