922打卡

1. 盛最多水的容器（11）

给定一个长度为 n 的整数数组 height 。有 n 条垂线，第 i 条线的两个端点是 (i, 0) 和 (i, height[i]) 

思想：双指针

class Solution {
    public int maxArea(int[] height) {
   int left = 0;
        int right = height.length-1;
        int max =0;
        while (left<right){
            int h = Math.min(height[left],height[right]);
            int w = right-left;
            max = max< h*w ? h*w:max;
            if(height[left]<height[right]){
                left++;
            }else {
                right--;
            }
        }
        return max;
    }
}

　2. 整数转罗马数字（12）

class Solution {
    int[] values = {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};
    String[] symbols = {"M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"};

    public String intToRoman(int num) {
        StringBuffer res = new StringBuffer();
        for (int i = 0; i < values.length; i++) {
            int value = values[i];
            String symbol = symbols[i];
            while (num >= value) {
                num -= value;
                res.append(symbol);
            }
            if (num == 0) {
                break;
            }
        }
        return res.toString();
    }
}

3. 罗马数字转整数（13）

class Solution { private int getValue(char ch) {
        switch(ch) {
            case 'I': return 1;
            case 'V': return 5;
            case 'X': return 10;
            case 'L': return 50;
            case 'C': return 100;
            case 'D': return 500;
            case 'M': return 1000;
            default: return 0;
        }
    }


    public int romanToInt(String s) {
        int sum =0;
        int prenum = getValue(s.charAt(0));
        for (int i = 1; i <s.length() ; i++) {
            int num = getValue(s.charAt(i));
            if (prenum>=num){
                sum+=prenum;
            }else {
                sum-=prenum;
            }
            prenum=num;
        }
        sum+=prenum;
        return sum;
    }
}

4.最长公共前缀(14)

class Solution {
   
    public String longestCommonPrefix(String[] strs) {
        if (strs==null||strs.length==0)
            return "";
        int minlen =Integer.MAX_VALUE;
        for (int i = 0; i <strs.length ; i++) {
            minlen = Math.min(strs[i].length(),minlen);
        }
        int low  = 0;
        int high = minlen;
        while (low<high){
            int mid = (high-low+1)/2+low;
            if(getPrefix(strs,mid)){
                low=mid;}
            else {high=mid-1;}
        }
        return strs[0].substring(0,low);

    }

    private boolean getPrefix(String[] strs, int length) {
        String str0 = strs[0].substring(0, length);
        int count = strs.length;
        for (int i = 1; i < count; i++) {
            String str = strs[i];
            for (int j = 0; j < length; j++) {
                if (str0.charAt(j) != str.charAt(j)) {
                    return false;
                }
            }
        }
        return true;
    }
}