[Swift]LeetCode724. 尋找數組的中心索引 | Find Pivot Index

時間 2019-11-11

標籤 swift leetcode724 leetcode 尋找數組中心索引 pivot index 欄目 Swift 简体版

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Given an array of integers nums, write a method that returns the "pivot" index of this array.git

We define the pivot index as the index where the sum of the numbers to the left of the index is equal to the sum of the numbers to the right of the index.github

If no such index exists, we should return -1. If there are multiple pivot indexes, you should return the left-most pivot index.數組

Example 1:微信

Input: 
nums = [1, 7, 3, 6, 5, 6]
Output: 3
Explanation: 
The sum of the numbers to the left of index 3 (nums[3] = 6) is equal to the sum of numbers to the right of index 3.
Also, 3 is the first index where this occurs.

Example 2:app

Input: 
nums = [1, 2, 3]
Output: -1
Explanation: 
There is no index that satisfies the conditions in the problem statement.

Note:this

The length of nums will be in the range [0, 10000].
Each element nums[i] will be an integer in the range [-1000, 1000].

給定一個整數類型的數組 nums，請編寫一個可以返回數組「中心索引」的方法。spa

咱們是這樣定義數組中心索引的：數組中心索引的左側全部元素相加的和等於右側全部元素相加的和。code

若是數組不存在中心索引，那麼咱們應該返回 -1。若是數組有多箇中心索引，那麼咱們應該返回最靠近左邊的那一個。htm

示例 1:

輸入: 
nums = [1, 7, 3, 6, 5, 6]
輸出: 3
解釋: 
索引3 (nums[3] = 6) 的左側數之和(1 + 7 + 3 = 11)，與右側數之和(5 + 6 = 11)相等。
同時, 3 也是第一個符合要求的中心索引。

示例 2:

輸入: 
nums = [1, 2, 3]
輸出: -1
解釋: 
數組中不存在知足此條件的中心索引。

說明:

nums 的長度範圍爲 [0, 10000]。
任何一個 nums[i] 將會是一個範圍在 [-1000, 1000]的整數。

140ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {        
 3         if nums.count < 3 {
 4             return -1
 5         }
 6         
 7         var sum = 0
 8         for i in nums {
 9             sum += i
10         }
11         
12         var left = 0
13         var right = 0
14         
15         for i in 0..<nums.count {
16             
17             left = left + nums[i]
18             right = sum - left
19             
20             if left - nums[i] == right {
21                 return i
22             }
23         }        
24         return -1
25     }
26 }

Runtime: 144 ms

Memory Usage: 19 MB

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         var sum:Int = nums.reduce(0,+)
 4         var curSum:Int = 0
 5         var n:Int = nums.count
 6         for i in 0..<n
 7         {
 8             if sum - nums[i] == 2 * curSum
 9             {
10                 return i
11             }
12             curSum += nums[i]
13         }
14         return -1
15     }
16 }

156ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         guard nums.count > 0 else {
 4             return -1
 5         }
 6         
 7         var leftSums: [Int] = [0]
 8         var rightSums: [Int] = [0]
 9         
10         var sum: Int = nums.first!
11         for i in 1..<nums.count {
12             leftSums.append(sum)
13             let num = nums[i]
14             sum += num
15         }
16         
17         sum = nums.last! 
18         for i in (0..<nums.count - 1).reversed() {
19             rightSums.append(sum)
20             sum += nums[i]
21         }
22         
23         rightSums = rightSums.reversed()
24         
25         for i in 0..<nums.count {
26             if leftSums[i] == rightSums[i] {
27                 return i
28             }
29         }
30         
31         return -1
32     }
33 }

160ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         var numsLength = nums.count
 4         guard numsLength != 0 else { return -1 }
 5         
 6         var fromRight = Array(repeating: Int.min, count: numsLength)
 7         fromRight[numsLength - 1] = nums[numsLength - 1]
 8         var i = numsLength - 2
 9         while i >= 0 {
10             fromRight[i] = nums[i] + fromRight[i + 1]
11             i -= 1
12         }
13         var fromLeft = 0
14         for i in 0..<(numsLength) {
15             var right = i + 1 < numsLength ? fromRight[i + 1] : 0
16             if fromLeft == right {
17                 return i
18             }
19             fromLeft += nums[i]
20             
21         }
22         return -1
23     }
24 }

172ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {        
 3         var leftSum: Int = 0
 4         var rightSum: Int = 0
 5         
 6         for num in nums {
 7             rightSum += num
 8         }
 9         
10         for (index, value) in nums.enumerated() {
11             
12             rightSum -= value
13             
14             if leftSum == rightSum {
15                 return index
16             } else {
17                 leftSum += value
18             }
19         }        
20         return -1
21     }
22 }

176ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         if nums.count <= 0{
 4             return -1
 5         }
 6         
 7         if nums.count == 1{
 8             return 0
 9         }
10         
11         // 計算和
12         var resunlt: Int = nums.reduce(0, +)
13         // 左邊
14         var left: Int = 0
15         
16         for i in 0...nums.count - 1 {
17             
18             resunlt -= nums[i]
19             if i == 0 {
20                 if resunlt == 0 {
21                     return i
22                 }
23             }else{
24                 left += nums[i - 1];
25                 if resunlt == left{
26                     return i
27                 }
28             }
29         }
30         return -1
31     }
32 }

177ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         var index = -1
 4         var numsSum = 0
 5         for i in nums {
 6             numsSum += i
 7         }
 8         var sum = 0
 9         for j in 0..<nums.count {
10             if (numsSum - nums[j])%2 == 0 {
11                 if sum == (numsSum - nums[j])/2 {
12                     index = j
13                     break
14                 }
15             }
16             sum += nums[j]
17         }
18         return index
19     }
20 }

180ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         if nums.isEmpty { return -1 }
 4         var sum = nums.reduce(0,+)
 5         var leftSum = 0
 6         for (index,element) in nums.enumerated() {
 7             if leftSum == (sum - leftSum) - element { return index }
 8             leftSum += element
 9         }
10         return -1
11     }
12 }

224ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         
 4         var total = nums.reduce(0, +)
 5         var sum = 0
 6         
 7         for i in 0..<nums.count {
 8             total -= nums[i]
 9             if sum == total {
10                 return i
11             } 
12             sum += nums[i]
13         }
14         return -1
15     }
16 }

225ms

 1 class Solution {
 2     func pivotIndex(_ nums: [Int]) -> Int {
 3         guard !nums.isEmpty else { return -1 }
 4 
 5         var prefixSums = Array(repeating: 0, count: nums.count)
 6 
 7         prefixSums[0] = nums[0]
 8         for (i, num) in nums.enumerated().dropFirst() {
 9             prefixSums[i] = prefixSums[i - 1] + num
10         }
11 
12         for (i, prefixSum) in prefixSums.enumerated() {
13             let leftSum = prefixSum - nums[i]
14             let rightSum = prefixSums.last! - nums[i] - leftSum
15 
16             if leftSum == rightSum {
17                 return i
18             }
19         }
20 
21         return -1
22     }
23 }