立贴！从今天开始刷题。

240. Search a 2D Matrix II

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

Integers in each row are sorted in ascending from left to right.
Integers in each column are sorted in ascending from top to bottom.
Example:

Consider the following matrix:

[
  [1,   4,  7, 11, 15],
  [2,   5,  8, 12, 19],
  [3,   6,  9, 16, 22],
  [10, 13, 14, 17, 24],
  [18, 21, 23, 26, 30]
]
Given target = 5, return true.

Given target = 20, return false.

用边界来缩小范围。

重刷了200. Number of Islands。

此题简单讲就是一次遍历所有节点。

发现节点是1，则island数量加1，然后调用另外一个递归函数去判断和这个1相邻的四个点是否也是小岛。

在递归函数内，将节点1换成0这样避免重复访问。

贴一下答案。

    public int numIslands(char[][] grid) {
        int count = 0;
        
        if(grid == null) return count;
        int y = grid.length;
        if(y==0) return count;
        int x = grid[0].length;
        if(x==0) return count;
        
        for(int j=0;j<y;j++){
            for(int i=0;i<x;i++){
                if(grid[j][i]=='1'){
                    count++;
                    marklands(grid,j,i);
                }
            }
        }
        return count;
    }
    public void marklands(char[][] grid,int j, int i)
    {
        int y = grid.length;
        int x = grid[0].length;
        if((j<0)||(i<0)) return;
        if((j>=y)||(i>=x)) return;
        
        if(grid[j][i]=='1'){
            grid[j][i] = '0';
            marklands(grid,j+1,i);
            marklands(grid,j-1,i);
            marklands(grid,j,i+1);
            marklands(grid,j,i-1);
        }
    }

答案没怎么美化，就凑合着看看。重刷此题，没看答案没看提示，完全自己做出。估计这题我可以不用再复习了。

543. Diameter of Binary Tree

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree
          1
         / \
        2   3
       / \     
      4   5   
Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.

此题还是看了一下答案才完全写对，需要好好总结一下。

937. Reorder Log Files

You have an array of logs.  Each log is a space delimited string of words.

For each log, the first word in each log is an alphanumeric identifier.  Then, either:

Each word after the identifier will consist only of lowercase letters, or;
Each word after the identifier will consist only of digits.
We will call these two varieties of logs letter-logs and digit-logs.  It is guaranteed that each log has at least one word after its identifier.

Reorder the logs so that all of the letter-logs come before any digit-log.  The letter-logs are ordered lexicographically ignoring identifier, with the identifier used in case of ties.  The digit-logs should be put in their original order.

Return the final order of the logs.



Example 1:

Input: ["a1 9 2 3 1","g1 act car","zo4 4 7","ab1 off key dog","a8 act zoo"]
Output: ["g1 act car","a8 act zoo","ab1 off key dog","a1 9 2 3 1","zo4 4 7"]


Note:

0 <= logs.length <= 100
3 <= logs[i].length <= 100
logs[i] is guaranteed to have an identifier, and a word after the identifier.

切割字符串，然后再判断是否是数字。

今天开始重刷题目

1. Two Sum
3. Longest Substring Without Repeating Characters