双指针|盛最多水的容器|移动0|接雨水

盛最多水的容器

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class Solution {
    public int maxArea(int[] height) {
        int n = height.length;
        int l = 0;
        int r = n - 1;
        int res = Integer.MIN_VALUE;
        while(l < r){
            int s = (r - l) * Math.min(height[l],height[r]);
            res = Math.max(res, s);
            if(height[l] < height[r]){
                l++;
            }
            else{
                r--;
            }
        }
        return res;
    }
}

移动0

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class Solution {
    public void moveZeroes(int[] nums) {
        int i = 0;
        int j = 0;
        while(i < nums.length){
            if(nums[i] != 0){
                nums[j] = nums[i];
                j++;
            }
            i++;
        }
        while(j < nums.length){
            nums[j] = 0;
            j++;
        }
    }
}

接雨水

思路一：一个数组存前缀最大值，一个数组存后缀最大值，遍历原数组求和。

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class Solution {
    public int trap(int[] height) {
        int n = height.length;
        int[] pre_max = new int[n];
        int[] suf_max = new int[n];
        int pre = Integer.MIN_VALUE;
        int suf = Integer.MIN_VALUE;

        int res = 0;
        for(int i = 0; i < n ; i ++){
            pre = Math.max(pre,height[i]);
            pre_max[i] = pre; 
        }
        for(int j = n-1; j >= 0 ; j--){
            suf = Math.max(suf,height[j]);
            suf_max[j] = suf; 
        }
        for(int k = 0; k < n; k++){
            int sum = (Math.min(pre_max[k],suf_max[k]) - height[k]);
            res += sum;
        }
        return res;
    }
}

思路二：双指针，左指针代表前缀最大值，右指针代表后缀最大值；利用左指针遍历；前缀小于后缀，加前缀，反之加后缀。

点击查看代码

class Solution {
    public int trap(int[] height) {
        int pre_max = 0;
        int suf_max = 0;
        int l = 0;
        int r = height.length - 1;

        int res = 0;
        while(l <= r){
            suf_max = Math.max(suf_max, height[r]);
            pre_max = Math.max(pre_max, height[l]);
            if(pre_max < suf_max){
                int sum = pre_max - height[l];
                res += sum;
                l++;
            }else{
                int sum = suf_max - height[r];
                res += sum;
                r--;
            }
        }
        return res;
    }
}