Leetcode 4.28 string

1. 38. Count and Say

　　就是对于前一个数，找出相同元素的个数，把个数和该元素存到新的string里。数量+字符

 class Solution {
     public String countAndSay(int n) {
         if( n == 1)
             return "1";
         StringBuffer sb = new StringBuffer();
         String str = countAndSay(n - 1);

         int count = 0;
         char c = '0';
         for( int i = 0; i < str.length(); i++ ){
             c = str.charAt(i);
             count = 1;
             while((i+1) < str.length() && str.charAt(i) == str.charAt(i+1)){
                 count++;
                 i++;
             }
             sb.append(count + "" + c);
         }
         return sb.toString();
     }
 }

2. 58. Length of Last Word

　　String.lastIndexOf();

;1 class Solution {
     public int lengthOfLastWord(String s) {
         s = s.trim();
         int lastIndex = s.lastIndexOf(' ') + 1;
         return s.length() - lastIndex;
     }
 }

3. 100. Same Tree

　　采用递归的方法，return中逻辑的运用。

 class Solution {
     public boolean isSameTree(TreeNode p, TreeNode q) {
         if( p == null & q == null) return true;
         if( p == null || q == null) return false;
         if(p.val == q.val)
             return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
         return false;
     }
 }