【Leetcode】Convert a Number to Hexadecima

题目链接：https://leetcode.com/problems/convert-a-number-to-hexadecimal/

题目：

Given an integer, write an algorithm to convert it to hexadecimal. For negative integer, two’s complement method is used.

Note:

1. All letters in hexadecimal (a-f) must be in lowercase.
2. The hexadecimal string must not contain extra leading 0s. If the number is zero, it is represented by a single zero character '0'; otherwise, the first character in the hexadecimal string will not be the zero character.
3. The given number is guaranteed to fit within the range of a 32-bit signed integer.
4. You must not use any method provided by the library which converts/formats the number to hex directly.

思路：

easy   负数在计算机中就是以补码形式存储的，所以对于负数无需特殊处理，按正数处理就可以

算法：

	public String toHex(int num) {
		if(num==0)
			return "0";
		StringBuilder ans = new StringBuilder();
		for(int i=0;i<32;i+=4){
			int a = (num>>i)&15;
			char token;
			if(a>9){
				token = (char)('a'+a-10);
			}else{
				token = (char)('0'+a);
			}
			ans.insert(0, token);
		}

		//消除前导零
		int s = 0;
		for(int i=0;i<32;i++){
			if(ans.charAt(i)!='0'){
				s= i;
				break;
			}
		}
		
		return ans.substring(s);
	}