Permutations@leetcode

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算法学习书籍：Introduction to Algorithms
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C++ 学习：The C++ Programming Language, 4th Edition
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创业：The Start-up of You: Adapt to the Future, Invest in Yourself, and Transform Your Career

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

» Solve this problem

刚开始想不用递归做，没想出来。后来参考了“水中的鱼” 的解法，用递归，关键点是要存储每一次的permutation，所以用值传递来传递参数，这样就不用把一堆string合并在一起再返回，也节约了运算时间。这类recursion还得多设计，熟练起来。