What is a search problem:
A solution to a search problem is a sequence of actions (a path) from s0 to a goal state. It is optimal if it has minimum sum of step costs.
Traditional Problem: Tower of Hanoi.
解释：搜索问题可以解释为从original state to target state构建的action-state 树，从而找到从s0 to st的最短路径。在树中，每个节点代表着states, 每条边代表着actions。
How to solve search problems:
tree search VS graph search: graph search has repeated states.
uninformed(/blind/brute-force) search VS informed search: uninformed search, no sense of closeness to the goal. 盲目搜索方法又叫非启发式搜索，是一种无信息搜索，一般只适用于求解比较简单的问题，盲目搜索通常是按预定的搜索策略进行搜索，而不会考虑到问题本身的特性。常用的盲目搜索有宽度优先搜索和深度优先搜索两种。
breadth-based search VS depth-base search:
The problem of tree search: failure to detect repeated states can lead to infinite loops when running Tree-Search, so there comes out graph search.
Queue VS stack:
queue: FIFO, first in first out;
stack: LIFO, last in first out;