42. Trapping Rain Water

class Solution {
    public int trap(int[] height) {
        int res = 0;
        int n = height.length;
        for (int i = 1; i < n - 1; i++) {
            int leftMax = 0, rightMax = 0;
            for (int j = i; j < n; j++) {
                rightMax = Math.max(rightMax, height[j]);
            }
            for (int j = i; j >= 0; j--) {
                leftMax = Math.max(leftMax, height[j]);
            }
            res += Math.min(leftMax, rightMax) - height[i];
        }
        return res;
    }
}

class Solution {
    public int trap(int[] height) {
        int n = height.length;
        if (n == 0) {
            return 0;
        }
        int[] leftMax = new int[n];
        int[] rightMax = new int[n];
        // init
        leftMax[0] = height[0];
        rightMax[n - 1] = height[n - 1];
        // calculate leftMax from left to right
        for (int i = 1; i < n; i++) {
            leftMax[i] = Math.max(leftMax[i - 1], height[i]);
        }
        // calculate rightMax from right to left
        for (int i = n - 2; i >= 0; i--) {
            rightMax[i] = Math.max(rightMax[i + 1], height[i]);
        }
        
        int res = 0;
        for (int i = 1; i < n - 1; i++) {
            res += Math.min(leftMax[i], rightMax[i]) - height[i];
        }
        return res;
    }
}

class Solution {
    public int trap(int[] height) {
        int n = height.length;
        if (n == 0) {
            return 0;
        }
        int leftMax = height[0];
        int rightMax = height[n - 1];
        int left = 0, right = n - 1;

        int res = 0;
        while (left <= right) {
            leftMax = Math.max(leftMax, height[left]);
            rightMax = Math.max(rightMax, height[right]);
            if (leftMax < rightMax) {
                res += leftMax - height[left];
                left++;
            } else {
                res += rightMax - height[right];
                right--;
            }
        }
        return res;
    }
}