48. search first index with Binary Search(possible duplicate array)

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This is a variation of 37. implement Binary Search (unique).

Your are given a sorted ascending array of number, but might have duplicates, you are asked to return the first index of a target number.

If not found return -1.

note

Please don't use Array.prototype.indexOf(), it is not our goal.

Let's try to solve this problem within 10 minutes.

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