Difference between writing ++i in the condition of a while loop and incrementing i inside the while loop

Admin 1
Viewed 166

I am writing the code as shown at the bottom of this question.

For the while loop part, I thought the following two codes are just the same

first one

                while (lIndex < rIndex && height[++lIndex] <= left) {
                    ans += (left - height[lIndex]);
                }

second one

                while (lIndex < rIndex && height[lIndex + 1] <= left) {
                    ans += (left - height[lIndex + 1]);
                    lIndex++;
                }

However, when I run the second one on system, There is an error of Time Limit Exceeding. Could someone explain the reason of this problem? Thanks for reading my question.

Original code:

    public int trap(int[] height) {
        if (height.length < 3) return 0;
        int ans = 0;
        int lIndex = 0;
        int rIndex = height.length - 1;
        
        // Find the first wall on each side
        while (lIndex < rIndex && height[lIndex] <= height[lIndex + 1]) lIndex++;
        while (lIndex < rIndex && height[rIndex] <= height[rIndex - 1]) rIndex--;
        
        while (lIndex < rIndex) {
            int left = height[lIndex];
            int right = height[rIndex];
            if (left <= right) {
                while (lIndex < rIndex && height[++lIndex] <= left) {
                    ans += (left - height[lIndex]);
                }
            }
            else {
                while (lIndex < rIndex && height[--rIndex] <= right) {
                    ans += right - height[rIndex];
                }
            }
            
        }
        return ans;
    }
while-loop java
1 Answers

In the first example, lIndex is incremented even when evaluation of the condition ends up with it being false. In other words, if the while loop body is executed n times, lIndex will be incremented n + 1 times.

In the second example, lIndex is only incremented with the rest of the body. So if the while loop body is executed n times, lIndex will be incremented n times.

Here's a very simple example of both, showing the difference:

public class Test {
    public static void main(String[] args) {
        int i = 0;
        while (++i < 3) {
            System.out.println("First loop iteration");
        }
        System.out.println("Value of i afterwards: " + i);

        int j = 0;
        while (j + 1 < 3) {
            System.out.println("Second loop iteration");
            j++;
        }
        System.out.println("Value of j afterwards: " + j);
    }
}

Output:

First loop iteration
First loop iteration
Value of i afterwards: 3
Second loop iteration
Second loop iteration
Value of j afterwards: 2

So both loops execute the body twice, but they end up with different counter values.

user_226560
Related