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Update 724._Find_Pivot_Index.md

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`@@ -73,3 +73,40 @@ class Solution(object):`
`73`	`73`	`return i`
`74`	`74`	`return -1`
`75`	`75`	```
	`76`	`+`
	`77`	`+> 思路 2`
	`78`	`+****- 时间复杂度: O(N)**- 空间复杂度: O(1)****`
	`79`	`+`
	`80`	`+后面发现其实空间只要O(1)就可以了,就是用left_sum, right_sum分别代表左右两边的和,初始化为0和sum(nums)`
	`81`	`+`
	`82`	+```python
	`83`	`+class Solution(object):`
	`84`	`+ def pivotIndex(self, nums):`
	`85`	`+ """`
	`86`	`+ :type nums: List[int]`
	`87`	`+ :rtype: int`
	`88`	`+ """`
	`89`	`+ left_sum, right_sum = 0, sum(nums)`
	`90`	`+ for idx, num in enumerate(nums):`
	`91`	`+ right_sum -= num`
	`92`	`+ if left_sum == right_sum:`
	`93`	`+ return idx`
	`94`	`+ left_sum += num`
	`95`	`+ return -1`
	`96`	+```
	`97`	`+`
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