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添加(0509.斐波那契数.md):增加javascript版本优化空间复杂度到O(1)解法

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Original file line number	Diff line number	Diff line change
`@@ -234,6 +234,7 @@ func fib(n int) int {`
`234`	`234`	`}`
`235`	`235`	```
`236`	`236`	`### Javascript`
	`237`	`+解法一`
`237`	`238`	```Javascript
`238`	`239`	`var fib = function(n) {`
`239`	`240`	`let dp = [0, 1]`
`@@ -244,6 +245,23 @@ var fib = function(n) {`
`244`	`245`	`return dp[n]`
`245`	`246`	`};`
`246`	`247`	```
	`248`	`+解法二:时间复杂度O(N),空间复杂度O(1)`
	`249`	+```Javascript
	`250`	`+var fib = function(n) {`
	`251`	`+ // 动规状态转移中,当前结果只依赖前两个元素的结果,所以只要两个变量代替dp数组记录状态过程。将空间复杂度降到O(1)`
	`252`	`+ let pre1 = 1`
	`253`	`+ let pre2 = 0`
	`254`	`+ let temp`
	`255`	`+ if (n === 0) return 0`
	`256`	`+ if (n === 1) return 1`
	`257`	`+ for(let i = 2; i <= n; i++) {`
	`258`	`+ temp = pre1`
	`259`	`+ pre1 = pre1 + pre2`
	`260`	`+ pre2 = temp`
	`261`	`+ }`
	`262`	`+ return pre1`
	`263`	`+};`
	`264`	+```
`247`	`265`
`248`	`266`	`TypeScript`
`249`	`267`

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