Javascript中提供了多种基本的数组合并的方法

reduce: builds up a value by repeated calling the iterator, passing in previous values; see the spec for the details; useful for summing the contents of an array and many other things

reduceRight: like reduce, but works in descending rather than ascending order

实际解决问题一 ：二维数组转一维数组，即数组的扁平化

var s = [[1,2],[3,4],[5,6]];

var r = s.reduce(function (p, c) {
    return p.concat(c);
});
console.log(r);
// [ 1, 2, 3, 4, 5, 6 ]

注解：reduce的callback中，p指代上一个元素，c指代当前元素。
第一次迭代：p -- > [1,2]      c -- > [3,4]
第二次迭代：p -- > [1,2,3,4]  c -- > [5,6]