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;
    }
}