立贴！从今天开始刷题。

向楼主学习

2 sum, map ;)

20. Valid Parentheses

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

    Open brackets must be closed by the same type of brackets.
    Open brackets must be closed in the correct order.

Note that an empty string is also considered valid.

Example 1:

Input: "()"
Output: true

Example 2:

Input: "()[]{}"
Output: true

Example 3:

Input: "(]"
Output: false

Example 4:

Input: "([)]"
Output: false

Example 5:

Input: "{[]}"
Output: true

此题用stack的办法，把对应的符号一个一个压进去。如果pop出的符号是相对应的符号而且stack为空，那就返回true。

155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

    push(x) -- Push element x onto stack.
    pop() -- Removes the element on top of the stack.
    top() -- Get the top element.
    getMin() -- Retrieve the minimum element in the stack.

Example:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

此题建2个stack一个存数据，另外一个存最小的数。然后和压进来的数字比较，如果比较小就作为新数压进来，否则就压老的最小的数字进站。

此题建2个stack一个存数据，另外一个存最小的数。然后和压进来的数字比较，如果比较小就作为新数压进来，否则就压老的最小的数字进站。

387. First Unique Character in a String

Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode",
return 2.

Note: You may assume the string contain only lowercase letters.

此题不难，但是有一个取巧的办法，建一个26个字母的数组。然后用arr[s.charAt(i)-'a']来访问index i的字符。

这样遍历一次就可以统计出哪个字母重复过了。

771. Jewels and Stones

You're given strings J representing the types of stones that are jewels, and S representing the stones you have.  Each character in S is a type of stone you have.  You want to know how many of the stones you have are also jewels.

The letters in J are guaranteed distinct, and all characters in J and S are letters. Letters are case sensitive, so "a" is considered a different type of stone from "A".

Example 1:

Input: J = "aA", S = "aAAbbbb"
Output: 3

Example 2:

Input: J = "z", S = "ZZ"
Output: 0

Note:

    S and J will consist of letters and have length at most 50.
    The characters in J are distinct.