leetcode 简单题（一）

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问题一

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

解决

上来不由分说，先来一个 For 再说😄

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func sumtotarget(nums []int, target int) []int {
 l := len(nums)
	for i, v := range nums {
		r := target - v
		for j:=i+1; j < l; j++ {
			if r == nums[j] {
				return []int{i, j}
			}
		}
	}
	return []int{}
}

完美成功运行了，哈哈。想想这样不行啊 O(n^2)。然后仔细想了一下，可以利用 map 的特性来处理，就有下面这一段代码。

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func sumtotarget(nums []int, target int) []int {
	numbers := make(map[int]int)
	for i, v := range nums {
		j, ok := numbers[v]
		numbers[target - v] = i
		if ok {
			return []int{j, i}
		}
	}

	return []int{}
}

第一刷，简单题都扛不住了。只能慢慢来了

问题二

Given a 32-bit signed integer, reverse digits of an integer.

Example 1:

Input: 123
Output: 321

Example 2:

Input: -123
Output: -321

Example 3:

Input: 120
Output: 21

Note:
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.

解决

注意的就是 int32 溢出的问题了

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func reverse(x int) int {
	var num int
	for  {
		n := x % 10
		if x == 0 && n == 0 {
			break
		}
		x = x / 10
		num = num * 10 + n
	}
	// -2147483648 到 2147483647
	if num > math.MaxInt32 || num < math.MinInt32 {
		return 0
	}
	return  num
}

问题三

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:

Input: 121
Output: true

Example 2:

Input: -121
Output: false

Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.
Example 3:

Input: 10
Output: false

Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

解决

这道题数字回文，其实和上面的反转是一个道理，就是比较反转后的数字和实参是否相等就可以了。

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func isPalindrome(x int) bool {
	if x < 0 {
		return false
	}
	var num int
	cache := x
	for  {
		n := x % 10

		if x == 0 && n == 0 {
			break
		}
		x = x / 10
		num = num * 10 + n
	}
	if cache == num {
		return true
	}

	return false
}