1. A symbol detector, for receiving a baseband signal to search for a maximum likelihood solution using a sphere decoding algorithm, comprising: a QR decomposer, performing a QR decomposition process on a channel response matrix to generate a Q matrix and an R matrix; a matrix transformer, coupled to the QR decomposer, generating an inner product matrix of the Q matrix and the baseband signal; a memory, coupled to the matrix transformer and the QR decomposer, buffering the R matrix and the inner product matrix; a scheduler, for determining a searching sequence of a plurality of independent branch missions originated from a search tree; and a plurality of Euclidean distance calculators, coupled to the scheduler and controlled by the scheduler to operate in parallel, wherein each comprise a plurality of calculation units cascaded in a pipeline structure to calculate the maximum likelihood solution based on the R matrix and the inner product matrix, wherein the channel response matrix comprises a plurality of column vectors, and wherein the QR decomposer sorts the column vectors in a particular order so that the generated R matrix comprising N full smallest diagonal elements arranged at the most lower right corner, and the other diagonal elements sorted from the upper-left to the lower-right in an incremental order, where N full is a variable denoting a full search depth.